ABSTRACTS

We present new measurements of the luminosity function (LF) of Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS) and the 2dF-SDSS LRG and Quasar (2SLAQ) survey. We have carefully quantified, and corrected for, uncertainties in the K and evolutionary corrections, differences in the colour selection methods, and the effects of photometric errors, thus ensuring we are studying the same galaxy population in both surveys. Using a limited subset of 6326 SDSS LRGs (with 0.17

We report the results of a systematic near-infrared spectroscopic survey using the Subaru, VLT and Keck Telescopes of a sample of high redshift Ultra-luminous Infrared Galaxies (ULIRGs) mainly composed of submillimeter-selected galaxies. Our observations span the restframe optical range containing nebular emission lines such as H-beta, [OIII] and [OII], which are essential for making robust diagnostics of the physical properties of these ULIRGs. Using the H-alpha/H-beta emission line ratios, we derive internal extinction estimates for these galaxies similar to those of local ULIRGs: A_V~2.9+/-0.5. Correcting the H-alpha estimates of the star formation rate for dust extinction using the Balmer decrement, results in rates which are consistent with those estimated from the far-infrared luminosity. The majority (>60%) of our sample show spectral features characteristic of AGN (although we note this partially reflects an observational bias in our sample), with ~65% exhibiting broad Balmer emission lines. A proportion of these sources show relatively low [OIII]/H-beta line ratios, which are similar to those of Narrow Line Seyfert 1 galaxies suggesting small mass black holes which are rapidly growing. In the subsample of our survey with both [OIII] and hard X-ray coverage, at least ~60% show an excess of [OIII] emission, by a factor of 5-10x, relative to the hard X-ray luminosity compared to the correlation between these two properties seen in Seyferts and QSOs locally. From our spectral diagnostics, we propose that the strong [OIII] emission in these galaxies arises from shocks in dense gaseous regions in this vigorously star-forming population. We caution that due to sensitivity and resolution limits, our sample is biased to strong line emitters and hence our results do not yet provide a complete view of the physical properties of the whole high-redshift ULIRG population.

We report deep K-band (18-27GHz) observations with the 100-m Green Bank Telescope of HCN(1-0) line emission towards the two submillimeter-selected galaxies (SMGs) SMMJ02399-0136 (z=2.81) and SMMJ16359+6612 (z=2.52). For both sources we have obtained spectra with channel-to-channel rms noise of <=0.5mJy, resulting in velocity-integrated line fluxes better than < 0.1 Jy km/s, although we do not detect either source. Such sensitive observations -- aided by gravitational lensing of the sources -- permit us to put upper limits of L_HCN(1-0) < 2x10^10 K km/s pc^2 on the intrinsic HCN(1-0) line luminosities of the two SMGs. The far-infrared (FIR) luminosities for all three SMGs with sensitive HCN(1-0) observations to date are found to be consistent with the tight FIR-HCN luminosity correlation observed in Galactic molecular clouds, quiescent spirals and (ultra) luminous infrared galaxies in the local Universe. Thus, the observed HCN luminosities remain in accordance with what is expected from the universal star formation efficiency per dense molecular gas mass implied by the aforementioned correlation, and more sensitive observations with today's large aperture radio telescopes hold the promise of detecting HCN(1-0) emission in similar objects in the distant Universe.

In this the first of a series of Letters, we present a description of the panchromatic data sets that have been acquired in the Extended Groth Strip region of the sky. Our survey, the All-wavelength Extended Groth strip International Survey (AEGIS), is intended to study the physical properties and evolutionary processes of galaxies at z ~ 1. It includes the following deep, wide-field imaging data sets: Chandra/ACIS X-ray (0.5-10 keV), GALEX ultraviolet (1200-2500A), CFHT/MegaCam Legacy Survey optical (3600-9000A), CFHT/CFH12K optical (4500-9000A), Hubble Space Telescope/ACS optical (4400-8500A), Palomar/WIRC near-infrared (1.2-2.2um), Spitzer/IRAC mid-infrared (3.6-8.0um), Spitzer/MIPS far-infrared (24-70um), and VLA radio continuum (6-20cm). In addition, this region of the sky has been targeted for extensive spectroscopy using the DEIMOS spectrograph on the Keck-2 10-m telescope. Our survey is compared to other large multiwavelength surveys in terms of depth and sky coverage.

We describe AEGIS20 - a radio survey of the Extended Groth Strip (EGS) conducted with the Very Large Array (VLA) at 1.4GHz. The resulting catalog contains 1,123 emitters and is sensitive to ultraluminous starbursts to z<=1.1, well matched to the redshift range of the DEEP2 spectroscopic survey in this region. We use stacking techniques to explore the uJy-level emission from a variety of galaxy populations selected via conventional criteria - Lyman-break galaxies (LBGs), distant red galaxies (DRGs), UV-selected galaxies and extremely red objects (EROs) - determining their properties as a function of color, magnitude and redshift and their extinction-free contributions to the history of star formation. We confirm the familiar pattern that the star-formation-rate (SFR) density, SFRD, rises by at least ~10x from z=0-1, although we note highly discrepant UV- and radio-based SFR estimates. Our radio-based SFRs become more difficult to interpret at z>1 where correcting for contamination by radio-loud active galactic nuclei (AGN) comes at the price of rejecting luminous starbursts. Whilst stacking radio images is a useful technique, accurate radio-based SFRs for z>>1 galaxies require precise redshifts and extraordinarily high-fidelity radio data to identify and remove accretion-related emission.

We report the first detection of CO(1-0) emission from a submillimeter-selected galaxy, using the Green Bank Telescope. We identify the line in the spectrum of SMM J13120+4242 as a broad emission feature at z=3.408, with Delta(V_FWHM)=1040 +/- 190 km/s. If the observed CO(1-0) line profile arises from a single object and not several merging objects, then the CO(4-3)/CO(1-0) brightness temperature ratio of ~0.26 suggests n(H_2) > 3-10 x 10^2 cm^-3 and the presence of sub-thermally excited gas. The integrated line flux implies a cold molecular gas mass M(H_2)=1.6 x 10^11 M_sun, comparable to the dynamical mass estimate and 4 times larger than the H_2 mass predicted from the CO(4-3) line assuming a brightness temperature ratio of 1.0. While our observations confirm that this submillimeter galaxy is massive and gas-rich, they also suggest that extrapolating gas masses from J_upper >= 3 transitions of CO leads to considerable uncertainties. We also report an upper limit to the mass of cold molecular gas in a second submillimeter galaxy, SMM J09431+4700, of M(H_2)< 4 x 10^10 M_sun.

We present two-dimensional spectroscopy covering the rest-frame wavelengths of strong optical emission lines in six luminous submillimetre galaxies at z=1.3-2.5. Together with HST ACS and NICMOS imaging we map the dynamics and morphologies of these systems on scales from 4-11kpc. Four of the systems show multiple components in their spatially-resolved spectra with average velocity offsets of ~180km/s across 8kpc in projection. From the ensemble properties from our survey and the literature, we estimate the typical dynamical masses of bright sub-mm galaxies as 5+/-3 * 10^11 Mo. This is similar to recent estimates of their stellar masses - suggesting that the dynamics of the central regions of these galaxies are baryon dominated, with a substantial fraction of those baryons in stars by the epoch of observation. Combining our dynamical mass estimates with stellar luminosities for this population we investigate whether submillimetre galaxies can evolve onto the Faber-Jackson relation for local ellipticals. Adopting a typical lifetime of tau_burst~300Myr for the submillimetre-luminous phase we find that the stellar populations of sub-mm galaxies should fade to place them on the Faber-Jackson relation, at M_K~-25.1. Furthermore, using the same starburst lifetime we correct the observed space density of submillimetre galaxies for the duty cycle to derive a volume density of the progenitors of ~1*10^-4 Mpc^-3. This is consistent with the space density of local luminous early-type galaxies with M_K~-25.1, indicating that submillimetre galaxies can evolve onto the scaling relations observed for local early-type galaxies, and the observed population at z~2 is then sufficient to account for the formation of the whole population of >~3 L^* ellipticals seen at z~0.

We describe the number counts and spatial distribution of 239 Distant Red Galaxies (DRGs), selected from the Early Data Release of the UKIDSS Ultra Deep Survey. The DRGs are identified by their very red infrared colours with (J-K)AB>1.3, selected over 0.62 sq degree to a 90% completeness limit of KAB~20.7. This is the first time a large sample of bright DRGs has been studied within a contiguous area, and we provide the first measurements of their number counts and clustering. The population shows strong angular clustering, intermediate between those of K-selected field galaxies and optical/infrared-selected Extremely Red Galaxies. Adopting the redshift distributions determined from other recent studies, we infer a high correlation length of r0~12 h-1 Mpc. Such strong clustering could imply that our galaxies are hosted by very massive dark matter halos, consistent with the progenitors of present-day L>L* elliptical galaxies.

We have exploited the large area coverage of the combined UKIDSS Ultra Deep Survey (UDS) and Subaru/XMM-Newton Deep Survey (SXDS) to search for bright Lyman-break galaxies (LBGs) at z >= 5. Using the available optical+near-infrared photometry to efficiently exclude low-redshift contaminants, we identify nine z >= 5 LBG candidates brighter than z'=25(AB) within the 0.6 square degree overlap region between the UDS early data release (EDR) and the optical coverage of the SXDS. Accounting for selection incompleteness, we estimate the corresponding surface density of z >= 5 LBGs with z'<=25(AB) to be 0.005+/-0.002 per square arcmin (18.5+/-3.5 per square degree). Modelling of the optical+near-infrared photometry constrains the candidates' redshifts to lie in the range 5.1 < z < 5.9, and provides estimates for their stellar masses. Although the stellar mass estimates are individually uncertain, a stacking analysis suggests that the typical stellar mass of the LBG candidates is >~5x10^10 Msun which, if confirmed, places them amongst the most massive galaxies currently known at z >= 5. The corresponding number density of massive LBGs at z >= 5 is found to be a factor of ~3 lower than the predicted density of suitable dark matter halos (i.e. M >~1-2x10^12 Msun) at these redshifts, and is therefore fully consistent with Lambda_CDM structure formation models. Moreover, it is found that recent galaxy formation models can also account for the existence of such massive galaxies at z >= 5. Finally, no evidence is found for the existence of LBGs with stellar masses in excess of 3x10^11 Msun at this epoch, despite the large co-moving volume surveyed.

We present panoramic Spitzer MIPS 24um observations covering 9x9Mpc (25'x25') fields around two massive clusters, Cl0024+16 and MS0451-03, at z=0.39 and z=0.55. Our observations cover a very wide range of environments within these clusters, from high-density regions around the cores out to the turn-around radius. Cross-correlating the mid-infrared catalogs with deep optical and near-infrared imaging of these fields, we investigate the optical/near-infrared colors of the mid-infrared sources. We find excesses of mid-infrared sources with optical/near-infrared colors expected of cluster members in the two clusters and test this selection using spectroscopically confirmed 24um members. The much more significant excess is associated with Cl0024+16, whereas MS0451-03 has comparatively few mid-infrared sources. The mid-infrared galaxy population in Cl0024+16 appears to be associated with dusty star-forming galaxies (typically redder than the general cluster population by up to A_V~1-2 mags) rather than emission from dusty tori around active galactic nuclei (AGN) in early-type hosts. The inferred total-infrared star-formation rates in Cl0024+16 are typically >5x greater than those found from a similar Halpha survey, indicating significant obscured activity in the cluster population. We find evidence for strong evolution of the level of dust-obscured star-formation in dense environments out to z=0.5, analogous to the rise in fraction of optically-selected star-forming galaxies seen in clusters and the field out to similar redshifts. However, there are clearly significant cluster-to-cluster variations in the populations of mid-infrared sources, probably reflecting differences in the intracluster media and recent dynamical evolution of these systems.

We present 350um observations of 15 Chapman et al. (2005) submillimeter galaxies (SMGs) with radio counterparts and optical redshifts. We detect 12 and obtain sensitive upper limits for 3, providing direct, precise measurements of their far-infrared luminosities and characteristic dust temperatures. With these, we verify the linear radio--far-infrared correlation at redshifts of z ~ 1-3 and luminosities of 10^11-10^13 L_sun, with a power-law index of 1.02+/-0.12, and rms scatter of 0.12 dex. However, either the the correlation constant 'q' or the dust emissivity 'beta' is lower than measured locally. The best fitting q ~ 2.14 is consistent with SMGs being predominantly starbust galaxies, without significant AGN contribution, at far-infrared wavelengths. Gas-to-dust mass ratios are estimated at 54+/-13 kappa(850um)/0.15 m^2/kg], depending on the absoption efficiency 'kappa', with intrinsic dispersion ~ 40% around the mean value. Dust temperatures consistent with 34.6+/-3K [beta/1.5]^-0.71, at z ~ 1.5-3.5, suggest that far-infrared photometric redshifts may be viable, and perhaps accurate to 10% <~ dz/(1+z), for up to 80% of the SMG population in this range, if the above temperature characterizes the full range of SMGs. However, observed temperature evolution of T_d ~ (1+z) is also plausible, and could result from selection effects. From the observed luminosity-temperature (L-T) relation, L ~ T_obs^(2.82+/-0.29), we derive scaling relations for dust mass vs. dust temperature, and identify expressions to inter-relate the observed quantities. These suggest that measurements at a single wavelength, in the far-infrared, submillimeter or radio wavebands, might constrain dust temperatures and far-infrared luminosities for most SMGs with redshifts at z ~ 0.5-4.

We describe the goals, design, and implementation of the UKIRT Infrared Deep Sky Survey (UKIDSS), a seven year sky survey which began in May 2005. It is a portfolio of five survey components covering various combinations of the filter set ZYJHK and H_2. The Large Area Survey, the Galactic Cluster Survey, and the Galactic Plane Survey cover approximately 7000 square degrees to a depth of K~18; the Deep Extragalactic Survey covers 35 square degrees to K~21, and the Ultra Deep Survey covers 0.77 square degrees to K~23. The prime aim of UKIDSS is to provide a long term astronomical legacy database; the design is however driven by a series of specific goals -- for example to find the nearest and faintest sub-stellar objects; to break the z=7 quasar barrier; to determine the epoch of re-ionisation; to determine the substellar mass function; to discover Population II brown dwarfs, if they exist; to measure the growth of structure from z=3 to the present day; to determine the epoch of spheroid formation; and to map the Milky Way through the dust, to several kpc. The data are being made available in a series of staged releases, the first of which (the "Early Data Release (EDR)") is described in Dye et al (2006). The data are immediately public to astronomers in all ESO member states, and available to the world after eighteen months. Before the formal survey began, UKIRT and the UKIDSS consortium collaborated in obtaining and analysing a series of small science verification (SV) projects to complete the commissioning of the camera. We show some results from these SV projects in order to demonstrate the likely power of the eventual complete survey.

We exploit the gravitational potential of massive cluster lenses to probe the emission line properties of six z=1 galaxies which appear as highly magnified luminous arcs. Using the GMOS integral field spectrograph together with detailed cluster lens models we reconstruct the intrinsic morphologies and two-dimensional velocity fields in these galaxies on scales corresponds to ~0.5kpc (unlensed) at z=1. Four of the galaxies have stable disk-like kinematics, whilst the other two resemble interacting or starburst galaxies. These galaxies lie close to the mean rest-frame I-band Tully-Fisher relation for nearby spirals suggesting a clear preference for hierarchical growth of structure. In the rest-frame B-band, the observations suggest 0.5+/-0.3 mag of brightening, consistent with increased star-formation activity at z=1. However, the galaxies with stable disk kinematics have more slowly rising rotation curves than expected from galaxies with similar surface brightness in the local Universe. We suggest that this may arise because the distant galaxies have lower bulge masses than their local counter-parts. Whilst this study is based on only six galaxies, the gain in flux and in spatial resolution achieved via gravitational magnification provides a much more detailed view of the high redshift Universe than possible with conventional surveys.

Extracting sources with low signal-to-noise from maps with structured background is a non-trivial task which has become important in studying the faint end of the submil- limetre number counts. In this article we study source extraction from submillimetre jiggle-maps from the Submillimetre Common-User Bolometer Array (SCUBA) using the Mexican Hat Wavelet (MHW), an isotropic wavelet technique. As a case study we use a large (11.8 sq. arcmin) jiggle-map of the galaxy cluster Abell 2218, with a 850-um 1-sigma r.m.s. sensitivity of 0.6-1 mJy. We show via simulations that MHW is a powerful tool for reliable extraction of low signal-to-noise sources from SCUBA jiggle-maps and nine sources are detected in the A2218 850-?m image. Three of these sources are identi- fied as images of a single background source with an unlensed flux of 0.8 mJy. Further, two single-imaged sources also have unlensed fluxes < 2 mJy, below the blank-field confusion limit. In this ultradeep map, the individual sources detected resolve nearly all of the extragalactic background light at 850um, and the deep data allow to put an upper limit of 44 sources per sq. arcmin to 0.2 mJy at 850um.

We present the analysis of the spectroscopic and photometric catalogues of 11 X-ray luminous clusters at z=0.07-0.16 from the Las Campanas / Anglo-Australian Telescope Rich Cluster Survey. Our spectroscopic dataset consists of over 1600 galaxy cluster members, of which two thirds are outside r_200. We assign cluster membership using a detailed mass model and expand on our previous work on the cluster colour-magnitude relation where membership was inferred statistically. We confirm that the modal colours of galaxies on the colour magnitude relation become progressively bluer with increasing radius and decreasing local galaxy density. Interpreted as an age effect, we hypothesize that these trends in galaxy colour should be reflected in mean Hdelta equivalent width. We confirm that passive galaxies in the cluster increase in Hdelta line strength as dHdelta / d r_p = 0.35 +/- 0.06. A variation of star formation rate, as measured by [OII], with increasing local density of the environment is discernible and is shown to be in broad agreement with previous studies from 2dFGRS and SDSS. We find that clusters at z~0.1 are less active than their higher redshift analogues. We also investigate unusual populations of blue and very red nonstarforming galaxies and we suggest that the former are likely to be the progenitors of galaxies which will lie on the colour-magnitude relation, while the colours of the latter possibly reflect dust reddening. The cluster galaxies at large radii consist of both backsplash ones and those that are infalling to the cluster for the first time. We make a comparison to the field population at z~0.1 and examine broad differences between the two populations. Individually, the clusters show significant variation in their galaxy populations which reflects their recent infall histories.

We present sub-arcsecond resolution IRAM PdBI interferometry of eight submillimeter galaxies at redshifts from 2 to 3.4, where we detect continuum at 1mm and/or CO lines at 3 and 1 mm. The CO 3-2/4-3 line profiles in five of the sources are double-peaked, indicative of orbital motion either in a single rotating disk or of a merger of two galaxies. The millimeter line and continuum emission is compact; we marginally resolve the sources or obtain tight upper limits to their intrinsic sizes in all cases. The median FWHM diameter for these sources and the previously resolved sources, SMMJ023952-0136 and SMMJ140104+0252 is less than or equal to 0.5" (4 kpc). The compactness of the sources does not support a scenario where the far-IR/submm emission comes from a cold, very extended dust distribution. These measurements clearly show that the submillimeter galaxies we have observed resemble scaled-up and more gas rich versions of the local Universe, ultra-luminous galaxy (ULIRG) population. Their central densities and potential well depths are much greater than in other redshift 2-3 galaxy samples studied so far. They are comparable to those of elliptical galaxies or massive bulges. The SMG properties fulfill the criteria of 'maximal' starbursts, in which most of the available initial gas reservoir of 10^10-10^11 solar masses is converted to stars on a few dynamical timescales.

The SCUBA HAlf Degree Extragalactic Survey (SHADES) is a major blank-field extragalactic sub-mm survey underway at the James Clerk Maxwell telescope.SHADES aims to cover half a square degree at 450+850um to a 4sigma depth of ~8mJy at 850um. Two fields are being observed, the Subaru/XMM-Newton Deep Field (SXDF) and the Lockman Hole East. The survey has 3 main aims to investigate: i) the cosmic history of massive dust enshrouded star-formation activity, ii) the clustering properties of sub-mm-selected galaxies in order to determine whether these objects could be progenitors of present-day massive ellipticals, and iii) the fraction of sub-mm-selected sources that harbour AGN. To achieve these aims requires the sub-mm data be combined with co-spatial information spanning the radio-to-X-ray frequency range, the resulting extensive multi-wavelength dataset providing complete photometric redshift information accurate to delta z <~0.5 and detailed SEDs for the vast majority of the sub-mm-selected sources. In this paper, the first of a series on SHADES, we present an overview of the motivation for the survey, describe the survey strategy and provide a detailed description of the primary data analysis pipeline. As of Feb. 2004, 720arcmin^2 had been mapped with SCUBA (about 40% of the anticipated final total) to a median 1sigma depth of 2.2mJy per beam at 850um (25mJy per beam at 450um), and a source density of 650+/-50 sources/deg^2 >3sigma at 850um, uncorrected for Eddington bias. A refined re-analysis of the original 8-mJy survey Lockman hole data was carried out in order to evaluate the new data reduction pipeline. Of the 17 most secure sources in the original sample, 12 have been re-confirmed, including 10 of the 11 for which radio IDs were previously secured.

The modest significance of most sources detected in current submm surveys can potentially compromise some analyses due to the inclusion of spurious sources in catalogues typically selected at >3.0-3.5sigma. Here, we develop and apply a dual-survey extraction technique to SCUBA and MAMBO images of the Lockman Hole. Cut above 5sigma, our catalogue of SMGs is more robust than previous samples, with a reduced likelihood of real, but faint SMGs (beneath and around the confusion limit) entering via superposition with noise. The effective flux limit of the survey is well matched to our deep 1.4-GHz image. The former is sensitive to luminous, dusty galaxies at extreme redshifts whilst the latter probes the z<3 regime. A high fraction of our robust SMGs (~80%) have radio counterparts which, given the ~10% contamination by spurious sources, suggests that very distant SMGs (z>>3) are unlikely to make up more than ~10% of the bright SMG population. This implies that almost all of the bright SMG population is amenable to study via the deepest current radio imaging. We use these radio counterparts to provide an empirical calibration of the positional uncertainty in SMG catalogues. We then go on to outline the acquisition of redshifts for radio-identified SMGs, from sample selection in the submm, to counterpart selection in the radio and optical/IR, to slit placement on spectrograph masks. We determine a median of z=2.05+/-0.41 from a sample of six secure redshifts for unambigious radio-identified submm sources and z=2.14+/-0.27 when we include submm sources with multiple radio counterparts and/or less reliable redshifts. These figures are consistent with previous estimates, suggesting that our knowledge of the median redshift of bright SMGs population has not been biased by the low significance of the source catalogues employed.

We present a multicolour catalogue of faint galaxies situated close to bright stars, V<15, with the aim of identifying high-redshift galaxies suitable for study with adaptive optics-equipped near-infrared imagers and spectrographs. The catalogue is constructed from archival calibration observations of UKIRT Faint Standard stars with the UFTI camera on UKIRT. We have analysed the deepest 16 fields from the archive to provide a catalogue of galaxies brighter than K~20.3 lying within 25" of the guide stars. We identify 111 objects in a total survey area of 8.7 sq. arcmin, of these 87 are classified as galaxies based on their light profiles in our ~0.5" median seeing K-band images. Of these, 12 galaxies have (J-K)>2.0 consistent with them lying at high-redshifts, z~2. These 12 very red galaxies have K-band magnitudes of K=18.1-20.1 and separations from the guide stars of 4-20" and hence are very well-suited to adaptive optics studies to investigate their morphologies and spectral properties on sub-kpc scales. We provide coordinates and JHK photometry for all catalogued objects.

We present the results from a submm survey of a sample of 23 giant Lya emitting nebulae in the overdensity at z=3.09 in the SA22 field. These objects, which have become known as Lya Blobs (LABs) have a diverse range of morphology and surface brightness, but the nature of their power source is unclear - with cooling flows and/or AGN/starburst ionised winds being possibilities. Using the SCUBA submm camera we measure the 850um flux of a sample of LABs, detecting four LABs at >3.5sigma individually, and a modest statistical detection of the full sample at about 3mJy. These fluxes correspond to bolometric luminosities in the ultraluminous regime, with star-formation rates of about 1e3 Msun/yr. We show there is a trend between Lya luminosity and bolometric output, which suggests that a galactic scale superwind generated from starbursts of age 10-100Myr may be responsible for the Lya emission. We estimate the star-formation rate density in SA22 to be >3 Msun/yr/Mpc^3 - greater than the field at this epoch, and note that there are now 7 submm galaxies in the SA22 structure, making this region the richest association of these intensely active galaxies. Finally we suggest that Lya haloes may be a common feature of the submm population in general, and have an important role in the heating and enrichment of the intergalactic medium.

We analyze deep X-ray, optical and mid-infrared Spitzer observations of the CDF-N/GOODS-N region to study 13 submillimeter-detected galaxies (SMGs) with spectroscopic redshifts (median z=2.2). We find a correlation between the estimated stellar and X-ray luminosity, implying that masses of the black holes may be related to the stellar masses of their host galaxies. Although the submillimeter emission implies that these galaxies are undergoing an epoch of intense star-formation, the Spitzer data reveal a massive stellar population already in place. These stellar masses are then compared to previously published black hole mass estimates derived from the X-ray luminosities under the assumption of Eddington-limit accretion. We find that the black hole masses for our high-redshift sample are approximately 1-2 orders of magnitude smaller than galaxies of comparable stellar mass in the local Universe. Although our estimates of black hole masses will increase if the accretion is sub-Eddington, and our stellar masses will decrease if we assume a much younger stellar population or a different initial mass function, we find that only through a combination of effects is it possible to shift the high redshift galaxies such that they lie on the local relation. This suggests that the black holes need to grow substantially between z=2.2 and the present-day, with much of the black hole growth occurring after the current obscured, far-infrared luminous phase of activity which is likely associated with the formation of the spheroid. This interpretation supports a scenario where SMGs pass through a subsequent accretion-dominated phase, where they would appear as optically bright quasars.

We report results from a panoramic spectroscopic survey of 955 objects in the field of the rich cluster Cl0024+1654 (z~0.4), complementing the HST imaging presented in the first paper in this series. Combining with previous work, we compile a catalog of 1394 unique redshifts in the field of this cluster, including 486 cluster members spread across an area 10 Mpc in diameter. We examine the properties of a large sample of 104 cluster early types as a function of cluster radius and local density, using them as sensitive tracers of the various physical processes that may be responsible for galaxy evolution. By constructing the Fundamental Plane of Cl0024, we infer an evolution in the mean mass to light ratio of early types with respect to z=0 of < Log(M/Lv ) >= -0.14+/-0.02. We detect a significantly increased scatter in the relationship compared to that seen in local clusters. Moreover, we observe a clear radial trend in the mass to light ratios of individual early types, with the oldest galaxies located in the cluster core. Galaxies are apparently younger at larger radius, with E+S0s in the periphery having M/Lv ratios that nearly match values seen in the field at a similar redshift. The strong radial trend is seen even when the sample is restricted to a narrow range in galaxy mass. Independent spectral indicators used in combination reveal an abrupt interaction with the cluster environment which occurs near the virial radius, revealed by small bursts of star formation in a population of dim early-types, as well as by enhanced Balmer absorption for a set of larger E+S0s closer to the cluster core. We construct a simple infall model used to compare the timescales and strengths of the observed interactions. We examine the possibility that bursts of star formation are triggered when galaxies suffer shocks as they encounter the intra-cluster medium, or by the onset of galaxy harassment.

Deep SCUBA surveys have uncovered a large population of massive submm galaxies (SMGs) at z>~1. Although it is generally believed that these galaxies host intense star-formation activity, there is growing evidence that a substantial fraction also harbor an AGN. We present here possibly the strongest evidence for this viewpoint to date: the combination of ultra-deep X-ray observations (the 2 Ms CDF-N) and deep Keck spectroscopic data. We find that the majority (~75%) of the spectroscopically identified SMGs host AGN activity; the other ~25% have X-ray properties consistent with star formation (X-ray derived SFRs of ~1300-2700 solars/yr). The AGNs have properties generally consistent with those of nearby luminous AGNs (L_X~1E43-3E44) and the majority (~80%) are heavily obscured. We construct composite rest-frame 2-20 keV spectra for three different obscuration classes. An ~1 keV equivalent width FeK-alpha emission line is seen in the composite X-ray spectrum of the most heavily obscured AGNs (N_H>5x10^{23}), suggesting Compton-thick or near Compton-thick absorption. Even taking into account the effects of absorption, we find that the average X-ray-FIR ratio of the AGN-classified SMGs is approximately one order of magnitude below that found for typical quasars. This result suggests that intense star-formation activity dominates the bolometric output; however, we also explore whether the X-ray-FIR ratio is intrinsically less than that found for typical quasars and postulate that some SMGs may be AGN dominated. We investigate the growth of massive black holes, discuss prospects for deeper observations, and explore the potential of the next generation of X-ray observatories.

We have investigated the rest-frame optical and far-infrared properties of a sample of extremely bright candidate Lyman-break galaxies (LBG) identified in the Sloan Digital Sky Survey. Their high ultraviolet luminosities and lack of strong ultraviolet emission lines are suggestive of massive starbursts, although it is possible that they are more typical luminosity LBGs which have been highly magnified by strong gravitational lensing. Alternatively, they may be an unusual class of weak-lined quasars. If the ultraviolet and submillimetre properties of these objects mirror those of less luminous, starburst LBGs, then they should have detectable rest-frame far-infrared emission. However, our submm photometry fails to detect such emission, indicating that these systems are not merely scaled-up (either intrinsically or as a result of lensing) examples of typical LBGs. In addition we have searched for the morphological signatures of strong lensing, using high-resolution, near-infrared imaging, but we find none. Instead, near-infrared spectroscopy reveals that these systems are, in fact, a rare class of broad absorption-line (BAL) quasars.

We discuss the properties of the bright submillimeter source SMMJ14011+0252 at z=2.56 which lies behind the central regions of the z=0.25 lensing cluster A1835. This system has a complex optical morphology consisting of at least five separate components. We reassess the extensive multiwavelength observations of this system and find strong support for the suggestion that one of these five components represents a foreground galaxy. The spectral and morphological properties of the foreground galaxy indicate that it is a low-luminosity, passive early-type disk member of the A1835 cluster. We estimate the likely properties of the dark matter halo of this galaxy from its stellar distribution. Based on these estimates we suggest that, contrary to earlier claims, this foreground galaxy is unlikely to significantly magnify the background submillimeter source. Thus SMMJ14011+0252 probably represents an intrinsically luminous submillimeter galaxy.

We present an analysis of the submillimetre/X-ray properties of 19 X-ray absorbed, Compton-thin quasi-stellar objects (QSOs) selected to have luminosities and redshifts which represent the peak of cosmic QSO activity. i.e. ~L* objects at 11.5, the high star formation rates are consistent with a scenario in which the QSOs evolve to become local luminous elliptical galaxies. Combining these results with previously published data for X-ray unabsorbed QSOs and submillimetre-selected galaxies we propose the following evolutionary sequence: the forming galaxy is initially far-infrared luminous but X-ray weak similar to the sources discovered by the Submillimetre Common-User Bolometer Array; as the black hole and spheroid grow with time a point is reached when the central QSO becomes powerful enough to terminate the star formation and eject the bulk of the fuel supply (the Compton-thin absorbed QSO phase); this transition is followed by a period of unobscured QSO activity which subsequently declines to leave a quiescent spheroidal galaxy.

The tight relationship between the masses of black holes and galaxy spheroids in nearby galaxies implies a causal connection between the growth of these two components. Optically luminous quasars host the most prodigious accreting black holes in the Universe and can account for >30% of the total cosmological black-hole growth. As typical quasars are not, however, undergoing intense star formation and already host massive black holes [>10^8 Mo], there must have been an earlier pre-quasar phase when these black holes grew [mass range ~10^6-10^8 Mo]. The likely signature of this earlier stage is simultaneous black-hole growth and star formation in distant (i.e. z>1; >8 billion light years away) luminous galaxies. Here we report ultra-deep X-ray observations of distant star-forming galaxies that are bright at submillimetre wavelengths. We find that the black holes in these galaxies are growing almost continuously throughout periods of intense star formation. This activity appears to be more tightly associated with these galaxies than any other coeval galaxy populations. We show that the black-hole growth from these galaxies is consistent with that expected for the pre-quasar phase.

In this paper we present results from an IRAM Plateau de Bure millimetre-wave Interferometer (PdBI) survey for CO emission towards radio-detected submillimetre galaxies (SMGs) with known optical and near-infrared spectroscopic redshifts. Five sources in the redshift range z~1-3.5 were detected, nearly doubling the number of SMGs detected in CO. We summarise the properties of all 12 CO-detected SMGs, as well as 6 sources not detected in CO by our survey, and use this sample to explore the bulk physical properties of the SMG population as a whole. The median CO line luminosity of the SMGs is = (3.8+/-2.0)x 10^10 K km/s pc^2. Using a CO-to-H_2 conversion factor appropriate for starburst galaxies, this corresponds to a molecular gas mass = (3.0+/-1.6) x 10^10 Mo within a ~2kpc radius, about four times greater than the most luminous local ultraluminous infrared galaxies (ULIRGs) but comparable to that of the most extreme high-redshift radio galaxies and QSOs. The median CO fwhm linewidth is broad, = 780+/-320 km/s, and the SMGs often have double peaked line profiles, indicative of either a merger or a disk. From their median gas reservoirs (~3x10^10 Mo) and star-formation rates (>700 Mo/yr) we estimate a lower limit on the typical gas-depletion time scale of >40Myr in SMGs. This is marginally below the typical age expected for the starbursts in SMGs, and suggests that negative feedback processes may play an important role in prolonging the gas consumption time scale. We find a statistically-significant correlation between the far-infrared and CO luminosities of the SMGs which extends the observed correlation for local ULIRGs to higher luminosities and higher redshifts.

We present optical and near-infrared integral field spectroscopy of the SCUBA galaxy SMM J163650.43+405734.5 (ELAIS N2 850.4) at z=2.385. We combine Ly-alpha and H-alpha emission line maps and velocity structure with high resolution HST ACS and NICMOS imaging to probe the complex dynamics of this vigorous star-burst galaxy. The imaging data shows a complex morphology, consisting of at least three components separated by ~1" (8kpc) in projection. When combined with the H-alpha velocity field from UKIRT UIST IFU observations we identify two components whose redshifts are coincident with the systemic redshift, measured from previous CO observations, one of which shows signs of AGN activity. A third component is offset by 220+/-50km/s from the systemic velocity. The total star formation rate of the whole system (estimated from the narrow-line H-alpha and uncorrected for reddening) is 340+/-50Mo/yr. The Ly-alpha emission mapped by the GMOS IFU covers the complete galaxy and is offset by +270+/-40km/s from the systemic velocity. This velocity offset is comparable to that seen in rest-frame UV-selected galaxies at similar redshifts and usually interpreted as a star-burst driven wind. The extended structure of the Ly-alpha emission suggests that this wind is not a nuclear phenomenon, but is instead a galactic scale outflow. Our observations suggest that the vigorous activity in N2 850.4 is arising as a result of an interaction between at least two dynamically-distinct components, resulting in a strong starburst, a starburst-driven wind and actively-fuelled AGN activity.

We investigate a spectroscopic sample of 48 early-type (E+S0) galaxies in the rich cluster Abell 2390 (z=0.23) and 48 E+S0 galaxies from a previously published survey of Abell 2218 (z=0.18). The A2390 spectra are based on Multi Object Spectroscopy with MOSCA at the Calar Alto 3.5-m, complemented by ground-based and HST F555W and F814W observations. Our investigation spans a broad range in luminosity (-20.5>M_r>-23.0) and a wide FOV of 1.5x1.5 Mpc^2. Since the A2218 and A2390 samples are very similar, we can combine them and analyse a total of 96 E+S0 galaxies at z~0.2. We construct the Faber-Jackson relation (FJR) for all E+S0s and detect a modest luminosity evolution. The average offset from the local FJR in Gunn r-band is (dM_r)=0.32+/-0.22 mag. Less-massive galaxies show a trend for a larger evolution than more-massive galaxies. HST/WFPC2 surface brightness profile fits were used to derive structural parameters. For 34 E+S0 galaxies which enter the Fundamental Plane (FP) we deduce a mild evolution in Gunn r with a zero-point offset of 0.10+/-0.06, corresponding to 0.31+/-0.18mag. Elliptical and lenticular galaxies are uniformly distributed along the FP with a similar scatter of 0.1 dex. We find little evidence for differences between the populations of ellipticals and S0s. Lenticulars induce on average a larger evolution of 0.44+/-0.18 mag than ellipticals with 0.02+/-0.21 mag. The M/L ratios at z=0.2 are offset by (dlog(M/L_r))=-0.12+/-0.06 dex compared to those of Coma. Our results can be reconciled with a passive evolution of the stellar populations and a high formation redshift for the bulk of the stars in E+S0 galaxies. However, our findings are also consistent with the hierarchical formation picture for rich clusters.

We have obtained spectroscopic redshifts using the Keck-I telescope for a sample of 73 submillimeter (submm) galaxies, with a median 850um flux density of 5.7mJy, for which precise positions are available through their faint radio emission. The galaxies lie at redshifts out to z=3.6, with a median redshift of 2.2 and an interquartile range z=1.7-2.8. Modeling a purely submm flux-limited sample, based on the expected selection function for our radio-identified sample, suggests a median redshift of 2.3 with a redshift distribution remarkably similar to the optically- and radio-selected Quasars. The observed redshift distributions are similar for the AGN and starburst sub-samples. The median R_AB=24.6 for the sample. However, the dust-corrected ultraviolet (UV) luminosities of the galaxies rarely hint at their huge bolometric luminosities indicated by their radio/submm emission, underestimating the true luminosity by a median factor of ~100 for SMGs with pure starburst spectra. Radio and submm observations are thus essential to select the most luminous, high-redshift galaxies. The 850\mum, radio, and redshift data is used to estimate the dust temperatures, and characterize photometric redshifts. Using 450um measurements for a subset of our sample we confirm that the median dust temperature of Td=36+/-7K, derived assuming the local FIR-radio correlation applies at high redshift, is reasonable. Individual 450um detections are consistent with the local radio-FarIR relation holding at z~2. This median Td is lower than that estimated for similarly luminous IRAS 60um galaxies locally. We demonstrate that dust temperature variations make it impossible to estimate redshifts for individual submm galaxies using simple long-wavelength photometric methods to better than dz~1. We calculate total infrared and bolometric luminosities (the median infrared luminosity estimated from the radio is 8.5+7.4/-4.6 x 10^12 Lo), construct a luminosity function, and quantify the strong evolution of the submm population across z=0.5-3.5, relative to local IRAS galaxies. We use the bolometric luminosities and UV-spectral classifications to determine a lower limit to the active galactic nucleus (AGN) content of the population, and measure directly the varying contribution of highly-obscured, luminous galaxies to the luminosity density history of the Universe for the first time. We conclude that bright submm galaxies contribute a comparable star formation density to Lyman-break galaxies at z=2-3 and including galaxies below our submm flux limit this population may be the dominant site of massive star formation at this epoch. The rapid evolution of submm galaxies and QSO populations contrasts with that seen in bolometrically lower luminosity galaxy samples selected in the restframe UV, and suggests a close link between submm galaxies and the formation and evolution of the galactic halos which host QSOs.

We present optical and near-infrared photometry for a sample of 96 dusty, far-infrared luminous galaxies. We have precise spectroscopic redshifts for all these galaxies yielding a median redshift of =2.2. The majority, 78, are submillimeter-detected galaxies lying at z=0.2-3.6, while the remaining 18 are optically-faint uJy radio galaxies at z=0.9-3.4 which are proposed to be similarly luminous, dusty galaxies whose dust emission is too hot to be detected in the submillimeter waveband. We compare the photometric and morphological properties of these distant, ultraluminous galaxies to local samples of dusty, luminous galaxies. We confirm that spectroscopically-identified far-infrared luminous galaxies at z>1 display a wide variety in their optical-near-infrared and near-infrared colors, with only a modest proportion red enough to classify as unusually red. We show that on average luminous, high-redshift dusty galaxies are both brighter and redder in restframe optical passbands than comparable samples of UV-selected star forming galaxies at similar redshifts. Archival HST ACS imaging of 20 of our galaxies demonstrates both morphological indications of mergers and interactions, which may have triggered their luminous far-infrared activity, and structured dust distributions within these galaxies. We derive a near-infrared Hubble diagram for far-infrared luminous galaxies. This shows that this population is typically fainter than high luminosity radio galaxies at similar redshifts and exhibit ignificantly more scatter in their K-band magnitudes. The restframe optical luminosities of the far-infrared luminous population are comparable to those of local ultraluminous infrared galaxies, although their far-infrared luminosities are several times higher. The typical extinction-corrected optical luminosity of the high-redshift population, assuming passive evolution, provides a good fit to the bright end of the luminosity function of luminous spheroidal galaxies seen in rich clusters at intermediate redshifts. This adds to the growing body of evidence showing that these high redshift, far-infrared luminous sources identify star-formation and AGN fueling events in the early life of massive galaxies in the Universe.

We present near-infrared spectroscopy and narrow-band imaging at the wavelength of redshifted H-alpha for a sample of 30 high-redshift, far-infrared luminous galaxies. This sample is selected from surveys in the sub-millimeter, millimeter and radio wavebands and has complete redshift coverage with a median redshift of z~2.4. We use our data to measure the H-alpha properties of these systems and to gauge the prevalence of AGN in these galaxies through their [NII]/H-alpha ratios and H-alpha line widths. Removing obvious AGN, we find that the predicted H-alpha star formation rates in this diverse population are suppressed (by a factor of ~10) compared to those derived from their far-infrared luminosities. Using the AGN indicators provided by our near-infrared spectra we estimate that AGN are present in at least 40% of the galaxies in our sample. To further investigate this we construct a composite rest-frame spectrum for both the entire sample and for those galaxies which individually show no signs of nuclear activity. We find [NII]/H-alpha ratios for both composite spectra which suggest that the energy output of the galaxies is star-formation- rather than AGN-dominated. However, we also find that the H-alpha line in the composite non-AGN spectrum is best fit with an underlying broad line component with a narrow/broad flux ratio of 0.45+/-0.20. The median H-alpha line width for our sample (removing obvious AGN) is 400+/-70 km/s (FWHM) and the typical spatial extent of the H-alpha emission in our narrow-band observations is <4-8kpc which indicates a dynamical mass of 1-2x10^11 Mo with corresponding dynamical times of 10-20 Myrs. Using both high-resolution imaging and spectroscopically identified velocity offsets we find that seven of the far-infrared luminous galaxies have companions, suggesting that they are undergoing interactions/mergers and from their relative velocities we can determine a dynamical mass of 1.5+/-0.9x10^11 Mo. These measurements are comparable to millimetre CO estimates for the dynamical masses of these systems on similar scales, and larger than recent estimates of the dynamical masses of UV-selected galaxies at similar redshifts derived in an identical manner. Using the [NII]/H-alpha index to predict abundances we investigate the Luminosity-Metallicity relation for these galaxies and find that many have metallicities consistent with UV selected high-redshift galaxies and slightly lower than local luminous infrared galaxies and ellipticals (although we caution that our metallicity estimates have possible systematic uncertainties). We also compared our H-alpha and far-infrared luminosities with deep Chandra observations of a subset of our survey fields and use these data to further assess their AGN content. We conclude that these high-redshift, far-infrared luminous galaxies represent a population of massive, metal-rich, merging systems with high instantaneous star formation rates, strong dust obscuration and actively-fuelled AGN which are likely to be the progenitors of massive local ellipticals.

We compare high-resolution optical and radio imaging of 12 luminous submillimeter (submm) galaxies at a median z=2.2+/-0.2 observed with Hubble Space Telescope (HST) and the MERLIN and VLA radio interferometers at comparable spatial resolution, ~0.3" (~2kpc). The radio emission is used as a tracer of the likely far-infrared morphology of these dusty, luminous galaxies. In ~30% of the sample the radio emission appears unresolved at this spatial scale, suggesting that the power source is compact and may either be an obscured AGN or a compact nuclear starburst. However, in the majority of the galaxies, ~70% (8/12), we find that the radio emission is resolved by MERLIN/VLA on scales of ~1" (~10kpc). For these galaxies we also find that the radio morphologies are often broadly similar to their restframe UV emission traced by our HST imaging. To assess whether the radio emission may be extended on even larger scales, >>1", resolved out by the MERLIN+VLA synthesized images, we compare VLA B-array (5-" beam) to VLA A-array (1.5-" beam) fluxes for a sample of 50 uJy radio sources, including 5 submm galaxies. The submm galaxies have comparable fluxes at these resolutions and we conclude that the typical radio emitting region in these galaxies are unlikely to be much larger than ~1" (~10kpc). We discuss the probable mechanisms for the extended emission in these galaxies and conclude that their luminous radio and submm emission arises from a large, spatially-extended starburst. The median star formation rates for these galaxies are ~1700Mo/yr (M>0.1Mo) occuring within regions with typical sizes of ~40kpc^2, giving a star formation density of 45 Mo/yr/kpc^2. Such vigorous and extended starburst appear to be uniquely associated with the submm population. A more detailed comparison of the distribution of UV and radio emission in these systems shows that the broad similarities on large scales are not carried through to smaller scales, where there is rarely a one-to-one correspondance between the structures seen in the two wavebands. We interpret these differences as resulting from highly structured internal obscuration within the submm galaxies, suggesting that their vigorous activity is producing wind-blown channels through their obscuring dust clouds. If correct this underlines the difficulty of using UV morphologies to understand structural properties of this population and also may explain the surprising frequency of Ly-alpha emission in the spectra of these very dusty galaxies.

We present high-resolution mass reconstructions for five massive cluster-lenses spanning a redshift range from z = 0.18-0.57 utilizing archival Hubble Space Telescope data and applying galaxy-galaxy lensing techniques. These detailed mass models were obtained by combining constraints from the observed strong and weak lensing regimes. Quantifying the local weak distortions in the shear maps in terms of perturbations induced by the presence of galaxy halos around individual bright early-type cluster member galaxies, we estimate the fraction of mass in the central regions of these clusters that can be associated with small scale mass clumps. This technique enables us to directly map the substructure in the mass range 10^11-10^12.5 solar masses which we associate with galaxy-scale sub-halos. The determination of the mass spectrum of substructure in the inner regions of these clusters is presented. Constraints are thereby obtained on the masses, mass-to-light ratios and truncation radii for these sub-halos. We find that the fraction of total cluster mass associated with individual sub-halos within the inner 0.5-0.8 h^-1 Mpc of these clusters ranges from 10-20%. Our results have important implications for the survival and evolution of substructure in high density cluster cores and are consistent with the theoretical picture of tidal stripping of galaxy-scale halos in high-density cluster environments as expected in hierarchical Cold Dark Matter dominated structure formation scenarios.

The massive cluster of galaxies Abell 2219 (z = 0.228) was observed at 14.3um with the Infrared Space Observatory and results were published by Barvainis et al. (1999). These observations have been reanalyzed using a method specifically designed for the detection of faint sources that had been applied to other clusters. Five new sources were detected and the resulting cumulative total of ten sources all have optical counterparts. The mid-infrared sources are identified with three cluster members, three foreground galaxies, an Extremely Red Object, a star and two galaxies of unknown redshift. The spectral energy distributions (SEDs) of the galaxies are fit with models from a selection, using the program GRASIL. Best-fits are obtained, in general, with models of galaxies with ongoing star formation. For three cluster members the infrared luminosities derived from the model SEDs are between ~5.7x10^10 Lo and 1.4x10^11 Lo, corresponding to infrared star formation rates between 10 and 24 Mo/yr. The two cluster galaxies that have optical classifications are in the Butcher-Oemler region of the color-magnitude diagramme. The three foreground galaxies have infrared luminosities between 1.5x10^10 Lo and 9.4x10^10 Lo yielding infrared star formation rates between 3 and 16 Mo/yr. Two of the foreground galaxies are located in two foreground galaxy enhancements (Boschin et al. 2004). Including Abell 2219, six distant clusters of galaxies have been mapped with ISOCAM and luminous infrared galaxies (LIRGs) have been found in three of them. The presence of LIRGs in Abell 2219 strengthens the association between luminous infrared galaxies in clusters and recent or ongoing cluster merger activity.

We present the results of observations from the IRAM array of the submm galaxy SMMJ16359+6612 lying at z=2.516 behind the massive cluster A2218. The foreground gravitational lens produces 3 images with a total magnification of 45 of this faint submm galaxy, which has an intrinsic submm flux of f_850um=0.8mJy placing it below the confusion limit of blank-field surveys. The substantial magnification provides a rare opportunity to probe the nature of a distant sub-mJy submm-selected galaxy, part of the population which produces the bulk of the submm cosmic far-infrared background. Our observations detect the CO(3-2) line in all 3 images, as well as the CO(7-6) line and the dust continuum at 1.3mm for the brightest image. The CO(3-2) velocity profile displays a double-peak profile which is well fit by two Gaussians with FWHM of 220km/s and separated by 280km/s. We estimate the dynamical mass of the system to be ~1.5x10^10 Mo and an H2 gas mass of 2.6x10^9 Mo. We identify a spatial offset of ~1" between the two CO(3-2) velocity components, modeling of which indicates that the offset corresponds to just ~3kpc in projection at z=2.5. The spatial and velocity properties of these two components are closely related to features detected in previously published Halpha spectroscopy. We conclude that this source is likely to be a compact merger of 2 fairly typical Ly-break galaxies with a maximal separation between the two nuclei of ~3kpc. This system is much less luminous and massive than other high-z submm galaxies studied to date, but it bears a close similarity to similarly luminous, dusty starburst resulting from lower-mass mergers in the local Universe.

We have formed ``composite spectra'' by combining the integrated-light spectra of individual galaxies in 8 intermediate-redshift and 12 low-redshift clusters of galaxies. Because these composite spectra have much higher signal-to-noise ratios than individual galaxy spectra, they are particularly useful in quantifying general trends in star formation for galaxy populations in distant clusters, z > 0.3. By measuring diagnostic features that represent stellar populations of very different ages, a grand-composite spectrum can reflect the fractions of those populations as accurately as if excellent spectral measurements were available for each galaxy. Measuring the equivalent widths of spectral features in composite spectra is especially well-suited for comparing cosmic variance of star formation in clusters at a given redshift, or comparing clusters over a range of redshifts. When we do this we find that [O II] emission and especially Balmer absorption is strong in each of our intermediate-redshift clusters, and completely separable from a sample of 12 present-epoch clusters, where these features are weak. Specifically, we show by comparing to the H-delta strengths of present-epoch populations of continuously star-forming galaxies that the higher-redshift samples must contain a much higher fraction of starburst galaxies than are found today in any environment.

We report results from a comprehensive follow-up observing campaign of the afterglow of GRB 030226, including VLT spectroscopy, VLT polarimetry, and Chandra X-ray observations. In addition, we present BOOTES-1 wide-field observations at the time of the occurrence of the burst. First observations at ESO started 0.2 days after the event when the GRB afterglow was at a magnitude of R~19 and continued until the afterglow had faded below the detection threshold (R>26). No underlying host galaxy was found. The optical light curve shows a break around 0.8 days after the burst, which is achromatic within the observational errors, supporting the view that it was due to a jetted explosion. Close to the break time the degree of linear polarization of the afterglow light was less than 1.1%, which favors a uniform jet model rather than a structured one. VLT spectra show two absorption line systems at redshifts z=1.962+/-0.001 and at z=1.986+/-0.001, placing the lower limit for the redshift of the GRB close to 2. We emphasize that the kinematics and the composition of the absorbing clouds responsible for these line systems is very similar to those observed in the afterglow of GRB 021004. This corroborates the picture in which at least some GRBs are physically related to the explosion of a Wolf-Rayet star.

We employ panoramic, multicolour (BRz') and narrow-band H-alpha imaging of the cluster Cl0024.0+1652 (z=0.39) from Subaru covering a ~30 arcmin field, to determine cluster membership and star formation rates for a large sample of galaxies across a wide field in the cluster, ~10 Mpc. We use photometric redshifts to identify cluster members, and statistically correct for the residual field contamination using similar data from the Subaru Deep Field. We detect over 500 galaxies in narrow-band emission, with broad-band colours consistent with them lying at z~0.39 Using this sample we determine the H-alpha luminosity function within the cluster and find that its form is approximately independent of local density, and is consistent with that seen in the intermediate redshift field population. This suggests that any density-dependent physical mechanisms which alter the star formation rate must leave the H-alpha luminosity function unchanged; this is possible if the time-scale for star formation to cease completely is short compared with a Hubble time. Such short time-scale transformations are also supported by the presence of a population with late-type morphologies but no detectable H-alpha emission. The fraction of blue galaxies, and the fraction of galaxies detected in H-alpha, decreases strongly with increasing galaxy density in a manner which is qualitatively similar to that seen at lower redshifts. This trend is significantly steeper than the trend with galaxy morphology observed from a panoramic Hubble Space Telescope image of this cluster; this suggests that the physical mechanisms responsible for transformations in morphology and star formation rates may be partially independent. Finally, we compare our data with similar data on clusters spanning a range of redshifts from z=0.2-0.8 and find little evidence for a trend in the total amount of star formation in clusters with redshift. Although the data can accommodate strong evolution, the scatter from cluster to cluster at fixed redshift is of a comparable magnitude.

We present the weak lensing methodology applied to our multi-colour imaging survey of X-ray luminous galaxy clusters conducted with the wide field CFH12k camera. This method, which is converting a fully reduced CFH12k image into cluster mass constraints, is done in two steps that we explain in detail: (1) determination of the "true" shape of faint (lensed) galaxies which involves: object detection strategy, point spread function (PSF) determination, galaxy shapes measurements with errors; (2) conversion of the faint galaxies catalogue into useful mass constraints which is done by different lensing techniques using 1D and/or 2D mass estimates. Finally, we compare the cluster mass model to the light distribution of cluster members as observed on our imaging data. To illustrate the method, we apply it to the well studied cluster Abell 1689 (z=0.184). In this cluster, we detect the gravitational shear signal to the edge of the image at 3 sigma significance. The two-dimensional mass reconstruction has a ~10 sigma significance mass peak centered on the brightest cluster galaxy. The weak lensing constraints are well fitted by a NFW mass profile with M_200 = 14.1^+6.3_-4.7x 10^14 Mo, and c = 3.5^+0.5/_-0.3 (chi^2=0.33), or by a power law (PL) profile with q=0.75+/-0.07 and theta_E = 14.6"+/-0.3" (chi^2=0.64). The mass-to-light ratio is found to be almost constant with radius with a mean value of M/L_R = 150 h (M/L)o. We compare these results to other weak lensing analyses of Abell 1689 presented in the literature and find good agreements in terms of the shear measurement as well as the final mass estimate.

We report spectroscopic redshifts for 18 microJy-radio galaxies at mean redshift of z=2.2 that are faint at both submmillimeter (submm) and optical wavelengths. While the radio fluxes of these galaxies could indicate far-infrared (far-IR) luminosities comparable to high-redshift submillimeter-selected galaxies (>10^12 Lsun), none are detected in the submm. We propose that this new population of galaxies represents an extension of the high-redshift submm galaxy population, but with hotter characteristic dust temperatures that shift the peak of their far-IR emission to shorter wavelengths, reducing the submm flux below the sensitivity of current instruments. Therefore, surveys in the submm waveband may miss up to half of the most luminous, dusty galaxies at z~2. Mid-infrared observations with Spitzer will be a powerful tool to test this hypothesis.

We present Spitzer observations in five wavebands between 3.6 and 24um of an unbiased sample of 9 luminous, dusty galaxies selected at 1200um by the MAMBO camera on the IRAM 30-m telescope, a population akin to the well-known submm or `SCUBA' galaxies (hereafter SMGs). Owing to the coarse resolution of submm/mm instrumentation, SMGs have traditionally been difficult to identify at other wavelengths. We compare our multi-wavelength catalogs to show that the overlap between 24 and 1200um must be close to complete at these flux levels. We find that all (4/4) of the most secure >=4sigma SMGs have robust >=4sigma counterparts at 1.4GHz, while the fraction drops to 7/9 using all >=3sigma SMGs. We show that combining mid-IR and marginal (>=3sigma) radio detections provides plausible identifications in the remaining cases, enabling us to identify the complete sample. Accretion onto an obscured central engine is betrayed by the shape of the mid-IR continuum emission for several sources, confirming Spitzer's potential to weed out active galaxies. We demonstrate the power of a S(24um)/S(8um) vs S(8um)/S(4.5um) color-color plot as a diagnostic for this purpose. However, we conclude that the majority (~75%) of SMGs have rest-frame mid-/far-IR SEDs commensurate with obscured starbursts. Sensitive 24-um observations are clearly a useful route to identify and characterize reliable counterparts to high-redshift far-IR-bright galaxies, complementing what is possible via deep radio imaging.

Using accurate positions from very deep radio observations to guide multi-object Keck spectroscopy, we have determined a substantially complete redshift distribution for very luminous, distant submillimeter(submm)-selected galaxies (SMGs). A sample of 73 redshifts for SMGs in 7 fields contains a surprisingly large number of `associations': systems of SMGs with Mpc-scale separations, and redshifts within 1200 km/s. This sample provides tentative evidence of strong clustering of SMGs at redshifts z~2-3 with a correlation length of about 6.9+/-2.1 Mpc/h, using a simple pair-counting approach that is appropriate to the small, sparse SMG samples. This is somewhat greater than the well-determined correlation lengths for both z~3 optical-ultraviolet(UV) color-selected Lyman-break galaxies (LBGs) and z~2 QSOs. This could indicate that SMGs trace the densest large-scale structures in the high-redshift Universe, and that they may be either be evolutionarily distinct from LBGs and QSOs, or subject to a more complex astrophysical bias.

The spectral energy distributions (SEDs) of dust-enshrouded galaxies with powerful restframe far-infrared(IR) emission have been constrained by a range of ground-based and space-borne surveys. The IRAS catalog provides a reasonably complete picture of the dust emission from nearby galaxies (at redshifts of order 0.1) that are typically less luminous than about 10^12Lo. However, at higher redshifts, the observational coverage from all existing far-IR and submillimeter (submm) surveys is much less complete. Here we investigate the SEDs of a new sample of high-redshift submm-selected galaxies (SMGs), for which redshifts are known, allowing us to estimate reliable luminosities and characteristic dust temperatures. We demonstrate that a wide range of SEDs is present in the population, and that a substantial number of luminous dusty galaxies with hotter dust temperatures could exist at similar redshifts (z~2-3), but remain undetected in existing submm surveys. These hotter galaxies could be responsible for about a third of the extragalactic IR background radiation at a wavelength of about 100um. The brightest of these galaxies would have far-IR luminosities of order 10^13Lo and dust temperatures of order 60 K. Galaxies up to an order of magnitude less luminous with similar SEDs will be easy to detect and identify in the deepest Spitzer observations of extragalactic fields at 24um.

We present a comprehensive space-based study of ten X-ray luminous galaxy clusters (Lx>8x10^44 erg/[0.1-2.4\,keV]) at z=0.2. Hubble Space Telescope (HST) observations reveal numerous gravitationally--lensed arcs for which we present four new spectroscopic redshifts, bringing the total to thirteen confirmed arcs in this cluster sample. The confirmed arcs reside in just half of the clusters; we thus obtain a firm lower limit on the fraction of clusters with a central projected mass density exceeding the critical density required for strong-lensing of 50%. We combine the multiple-image systems with the weakly--sheared background galaxies to model the total mass distribution in the cluster cores (R<500kpc). These models are complemented by high-resolution X-ray data from Chandra and used to develop quantitative criteria to classify the clusters as relaxed or unrelaxed. Formally, 30+/-20% of the clusters form a homogeneous sub-sample of relaxed clusters; the remaining 70+/-20% are unrelaxed and are a much more diverse population. Most of the clusters therefore appear to be experiencing a cluster-cluster merger, or relaxing after such an event. We also study the normalization and scatter of scaling relations between cluster mass, luminosity and temperature. The scatter in these relations is dominated by the unrelaxed clusters and is typically sigma~0.4. Most notably, we detect 2-3 times more scatter in the mass-temperature relation than theoretical simulations and models predict. The observed scatter is also asymmetric - the unrelaxed systems are systematically 40% hotter than the relaxed clusters at 2.5-sigma significance. This structural segregation should be a major concern for experiments designed to constrain cosmological parameters using galaxy clusters. Overall our results are consistent with a scenario of cluster-cluster merger induced boosts to cluster X-ray luminosities and temperatures.

We present observations of a remarkable submillimetre-selected galaxy, SMMJ16359+6612. This distant galaxy lies behind the core of a massive cluster of galaxies, A2218, and is gravitationally lensed by the foreground cluster into three discrete images which were identified in deep submillimetre maps of the cluster core at both 450 and 850micron. Subsequent follow-up using deep optical and NIR images identify a faint counterpart to each of the 3 images, with similar red optical--NIR colours and HST morphologies. By exploiting a detailed mass model for the cluster lens we estimate that the combined images of this galaxy are magnified by a factor of ~45, implying that this galaxy would have un-lensed magnitudes K_s=22.9 and I=26.1, and an un-lensed 850micron flux density of only 0.8mJy. Moreover, the highly constrained lens model predicted the redshift of SMMJ16359+6612 to be z=2.6+/-0.4. We confirm this estimate using deep optical and NIR Keck spectroscopy, measuring a redshift of z=2.516. SMMJ16359+6612 is the faintest submm-selected galaxy so far identified with a precise redshift. Thanks to the large gravitational magnification of this source, we identify 3 sub-components in this submm galaxy, which are also seen in the NIRSPEC data, arguing for either a strong dust (lane) absorption or a merger. Interestingly, there are 2 other highly-amplified galaxies at almost identical redshifts in this field (although neither is a strong submm emitter). The 3 galaxies lie within a ~100kpc region on the background sky, suggesting this submm galaxy is located in a dense high-redshift group.

The genesis of spheroids is central to our understanding of galaxy formation -- they are relatively simple systems, containing about half the stellar mass of the Universe. A major subset of spheroids, massive elliptical galaxies, are preferentially found in clusters where they exhibit old, coeval stellar populations suggesting that they formed synchronously at early epochs. Here we report SCUBA submillimeter imaging of a region around a z=1.8 X-ray selected QSO. The image reveals a remarkable 400 kiloparsec long chain of galaxies each with an obscured star-formation rate sufficiently high to build a massive spheroid in less than 1 Gyr. The large over-density of these galaxies relative to expectations for a random field implies they probably reside in a structure associated with the QSO. We suggest that this star formation is associated with galaxy mergers or encounters within the filament, such as those predicted by the popular hierarchical model of galaxy formation. Our observations suggest that strong absorption in the X-ray spectra of QSOs at high-redshifts may result from a veil of gas thrown up by a merger or merger-induced activity, rather than an orientation-dependent obscuring torus. It is argued that these systems are the precursors of elliptical galaxies found today in the core regions of all rich galaxy clusters.

We have studied the morphology-density relation and morphology-cluster-centric-radius relation using a volume-limited sample (0.05 < z < 0.1, Mr* < -20.5) of the Sloan Digital Sky Survey (SDSS) data. Major improvements compared with previous work are: (i) automated galaxy morphology classification capable of separating galaxies into four types; (ii) three-dimensional local galaxy density estimation; and (iii) the extension of the morphology-density relation into the field region. We found that the morphology-density and morphology-cluster-centric-radius relation in the SDSS data for both of our automated morphological classifiers, Cin and Tauto, as fractions of early-type galaxies increase and late-type galaxies decrease toward increasing local galaxy density. In addition, we found that there are two characteristic changes in both the morphology-density and the morphology-radius relations, suggesting that two different mechanisms are responsible for the relations. In the sparsest regions (below 1 Mpc-2 or outside of 1 virial radius), both relations become less noticeable, suggesting that the physical mechanisms responsible for galaxy morphological change require a denser environment. In the intermediate-density regions (density between 1 and 6 Mpc-2 or virial radius between 0.3 and 1), intermediate-type fractions increase toward denser regions, whereas late-disc fractions decrease. Considering that the median size of intermediate-type galaxies is smaller than that of late-disc galaxies, we propose that the mechanism is likely to stop star formation in late-disc galaxies, eventually turning them into intermediate-type galaxies after their outer discs and spiral arms become invisible as stars die. For example, ram-pressure stripping is one of the candidate mechanisms. In the densest regions (above 6 Mpc-2 or inside 0.3 virial radii), intermediate-type fractions decrease radically and early-type fractions increase in turn. This is a contrasting result to that in intermediate regions and it suggests that yet another mechanism is more responsible for the morphological change in these regions. We also compared the morphology-density relation from the SDSS (0.01

Rapid developments in near-infrared (NIR) arrays and adaptive optics systems have driven the development of wide-field and high-spatial-resolution, high-optical-quality NIR imagers and spectrographs, providing an unparalleled boost to NIR observations. Based around a 1024x1024 pixel^2 Hawaii-1 array, the Isaac Newton Group Red Imaging Device (INGRID) imager provides a field of view >16 arcmin^2 (at the Cassegrain focus) whilst Nyquist sampling the median summer seeing disc. When used in conjunction with the Nasmyth Adaptive Optics for Multi-Purpose Instrumentation (NAOMI) system and a second set of collimation optics, a high spatial resolution mode (0.04 arcsec/pixel) is offered, providing near-diffraction-limited imaging. INGRID uses an all-refractive design and employs a cold stop to reduce thermal background emission, critical to the performance as it is used on the non-infrared optimized 4.2-m William Herschel Telescope (WHT). We discuss the design and operation of INGRID and illustrate its performance by discussing commissioning observations of the cluster Abell 2218 and the spiral galaxies NGC 3351 and 1530.

We report on deep near-infrared (NIR) observations of submillimeter-selected galaxies (SMGs) with the Near Infrared Camera (NIRC) on the Keck I telescope. We have identified K-band candidate counterparts for 12 out of 15 sources in the SCUBA Cluster Lens Survey. Three SMGs remain non-detections with K-band limits of K>23 mag, corrected for lensing. Compensating for lensing we find a median magnitude of K=22+/-1 mag for the SMG population, but the range of NIR flux densities spans more than a factor of 400. For SMGs with confirmed counterparts based on accurate positions from radio, CO, and/or millimeter continuum interferometric observations, the median NIR color is J-K=2.6+/-0.6 mag. The NIR-bright SMGs (K<19 mag) have colors of J-K =~ 2 mag, while the faint SMGs tend to be extremely red in the NIR (J-K>3 mag). We argue that a color selection criterion of J-K>~3 mag can be used to help identify counterparts of SMGs that are undetected at optical and radio wavelengths. The number density of sources with J-K>3 mag is 5 arcmin^{-2} at K<22.5 mag, greater than that of SMGs with S(850um)>2 mJy. It is not clear if the excess represents less luminous infrared-bright galaxies with S(850um)<~2 mJy, or if the faint extremely red NIR galaxies represent a different population of sources that could be spatially related to the SMGs.

The most massive galaxies in the present-day Universe are found to lie in the centres of rich clusters. They have old, coeval stellar populations suggesting that the bulk of their stars must have formed at early epochs in spectacular starbursts - luminous phenomena at submillimetre wavelengths. The most popular model of galaxy formation predicts that these galaxies form in proto-clusters at high-density peaks in the early Universe. Such peaks are signposted by massive high-redshift radio galaxies. Here we report deep submillimetre mapping of seven high-redshift radio galaxies and their environments. These data confirm not only the presence of spatially extended massive star-formation activity in the radio galaxies themselves, but also in companion objects previously undetected at any wavelength. The prevalence, orientation, and inferred masses of these submillimetre companion galaxies suggest that we are witnessing the synchronous formation of the most luminous elliptical galaxies found today at the centres of rich galaxy clusters.

We present the results from a survey for Extremely Red Objects (EROs) in deep, high resolution optical images taken from the Hubble Space Telescope (HST) Medium Deep Survey. We have surveyed 35 deep F814W HST/WFPC2 fields in the near-infrared to a typical depth of K~20. From a total area of 206 arcmin^2 and to a limit of K=20.0 we identify 224 EROs ((1.14+/-0.08) arcmin^-2) with (I_{814}-K)=>4.0 and 83 ((0.41+/-0.05) arcmin^-2) with (I-K)=>5.0. We find that the slope of the number counts of the (I-K)=>4.0 EROs flattens beyond K~19, in line with results from previous surveys, and the typical colours of the EROs become redder beyond the break magnitude. We morphologically classify our ERO sample using visual and quantitative schemes and find that 35% of our sample exhibit clear disk components, 15% are disturbed or irregular, a further 30% are either spheroidal or compact and the remaining 20% are unclassifiable. Using a quantitative measure of morphology, we find that the ERO morphological distribution evolves across the break in their counts, such that low concentration (disk-like) galaxies decline. We relate the morphological and colour information for our EROs and conclude that those EROs morphologically classified as bulges do indeed possess SEDs consistent with passive stellar populations; while EROs with dusty star-forming SEDs are mostly associated with disk-like and peculiar galaxies. However, ~30% of disk EROs reside in the passive region of I/J/K colour-colour space. These could be either genuinely passive systems, lower redshift contaminants to the high-z ERO population, or systems with composite star-forming and passive SEDs.

The most massive galaxies in the present day universe are the giant ellipticals found in the centers of rich clusters. These have old, coeval stellar populations, suggesting they formed at high redshift, and are expected to host supermassive black holes (SMBHs). The recent detection of several high-redshift radio galaxies (HzRGs) at submm wavelengths confirms that indeed some massive galaxies may have formed the bulk of their stellar populations in spectacular dust-enshrouded starbursts at high-z. In this letter we compare sensitive Chandra X-ray images - which identify actively-fueled SMBHs - and submm observations - capable of detecting obscured activity in luminous galaxies at high redshift - of the environments of three HzRGs. These observations exhibit overdensities of X-ray sources in all 3 fields and a close correspondence between the Chandra and SCUBA populations. This suggests that both substantial star formation and nuclear activity may be occuring in these regions. We identify possible pairs of Chandra sources with each of 2 SCUBA sources, suggesting that their ultraluminous activity may be triggered by the interaction of two massive galaxies, each of which hosts an accreting SMBH. The presence of two SMBHs in the resulting remanent is predicted to produce a flattened stellar core in the galaxy, a morphological signature frequently seen in luminous cluster ellipticals. Hence the confirmation of pairs of Chandra sources within individual, luminous SCUBA galaxies would provide additional evidence that these galaxies at z~2-4 are the progenitors of the giant elliptical galaxies found in clusters at the present-day.

In this letter, we exploit the gravitational potential of the rich cluster A2218 as a magnifying glass. We demonstrate that the magnification due to the cluster allows us to observe distant background galaxies at a comparable level of detail to galaxies at z~0.1. Using the GMOS Integral Field Unit on Gemini North we observed the spatially-resolved [OII]3727 emission line spectrum for a lensed disk-galaxy at z=1.034. Using a detailed model for the cluster mass distribution, we are able to correct for the lensing by the cluster and reconstruct the source morphology. We find that the overall magnification is a factor of 4.92+/-0.15, and the rest-frame absolute I-band magnitude is M_I(rest) = -22.4+/-0.2, where the error bars include conservative estimates of the uncertainty in the source-plane reconstruction. The inclination-corrected circular velocity is 206+/-18km/s. The galaxy lies very close to the mean Tully-Fisher relation of present-day spirals. Although our results are based on a single object, they demonstrate that gravitational lensing can be viably used to make detailed studies of the evolution of the structure of distant field galaxies.

We present a comprehensive lensing analysis of the rich cluster Cl0024+1654 (z=0.395) based on panoramic sparse-sampled imaging conducted with the WFPC2 and STIS cameras on board the Hubble Space Telescope. By comparing higher fidelity signals in the limited STIS data with the wider field data available from WFPC2, we demonstrate an ability to detect reliably weak lensing signals to a cluster radius of ~5 / h_65 Mpc where the mean shear is around 1%. This enables us to study the distribution of dark matter with respect to the cluster light over an unprecedented range of cluster radius and environments. The projected mass distribution reveals a secondary concentration representing 30% of the overall cluster mass, which is also visible in the distribution of cluster member galaxies. We develop a method to derive the projected mass profile of the main cluster taking into account the influence of the secondary clump. We normalize the mass profile determined from the shear by assuming that background galaxies selected with I=23-26 have a redshift distribution statistically similar to that inferred photometrically in the Hubble Deep Fields (HDFs). The total mass within the central region of the cluster is independently determined from strong lensing constraints according to a detailed model which utilizes the multiply-imaged arc at z=1.675. Combining strong and weak constraints, we are able to probe the mass profile of the cluster on scales of 0.1 to 5 Mpc thus providing a valuable test of the universal form proposed by Navarro, Frenk \& White (1997) on large scales. A generalized power law fit indicates an asymptotic 3-D density distribution of rho=r^-n with n>2.4. An isothermal mass profile is therefore strongly rejected, whereas a NFW profile with M_200= 6.1+1.2/-1.1 10^14/h_65 Mo provides a good fit to the lensing data. We isolate cluster members according to their optical-near infrared colors; the red cluster light closely traces the dark matter with a mean mass-to-light ratio of M/L_K= 40+/-5 h_65 Mo/Lo. Similar profiles for mass and light on 1-5 Mpc scales are expected if cluster assembly is largely governed by infalling groups.

We present sensitive, high-resolution, X-ray imaging from Chandra of the high-redshift radio galaxy 4C 41.17 (z=3.8). Our 150-ks Chandra exposure detects strong X-ray emission from a point source coincident with the nucleus of the radio galaxy. In addition we identify extended X-ray emission with a luminosity ~1e45 erg/s covering a 100kpc (15'') diameter region around the radio galaxy. The extended X-ray emission follows the general distribution of radio emission in the radio lobes of this source, and the distribution of a giant Lyman-alpha emission line halo, while the spectrum of the X-ray emission is non-thermal and has a power law index consistent with that of the radio synchrotron. We conclude that the X-ray emission is most likely Inverse-Compton scattering of far-infrared photons from a relativistic electron population probably associated with past and current activity from the central object. Assuming an equipartition magnetic field the CMB energy density at z=3.8 can only account for at most 40% of the Inverse-Compton emission. Published submillimeter maps of 4C 41.17 have detected an apparently extended and extremely luminous far-infrared emission around the radio galaxy. We demonstrate that this photon component and its spatial distribution, in combination with the CMB can reproduce the observed X-ray luminosity. We propose that photo-ionization by these Inverse-Compton X-ray photons plays a significant role in this system, and provides a new physical feedback mechanism to preferentially affect the gas within the most massive halos at high redshift. This is the highest redshift example of extended X-ray emission around a radio galaxy currently known.

A significant fraction of the energy emitted in the early Universe came from very luminous galaxies that are largely hidden at optical wavelengths (because of interstellar dust grains); this energy now forms part of the cosmic background radiation at wavelengths near 1mm. These submillimetre (submm) galaxies were resolved from the background in 1997 but have been difficult to identify and study due to the poor spatial resolution of submm instruments. This has impeded the determination of their distances (or redshifts, z), a crucial element in understanding their nature and evolution. Here we report spectroscopic redshifts for ten representative submm galaxies that we identified reliably using high resolution radio observations. The median redshift for our sample is 2.4, with a quartile range of z = 1.9-2.8. The submm population therefore coexists with the peak activity of quasars, which are thought to be massive black holes in the process of accreting matter, suggesting a close relationship between the growth of massive black holes and luminous dusty galaxies. The space density of submm galaxies at z>2 is about 1000 times greater than that of similarly luminous galaxies in the present-day Universe, so they represent an important component of star formation at high redshifts.

We report the redshift measurement for the submillimeter selected galaxy SMMJ04431+0210 (N4) using the Near Infrared Spectrograph on the Keck-II telescope. The data show H-alpha, [NII]6583,6548, and [OIII]5007 lines at a redshift of z=2.51. The high nuclear [NII]/H-alpha line ratio is consistent with a LINER or Type-II AGN. The H-alpha emission is spatially resolved, suggesting the presence of significant star-forming activity outside the nucleus. From imaging with the Near Infrared Camera on the Keck-I telescope, we find an extremely red near-infrared color of J-K=3.2 for N4. Follow-up redshifted CO(3-2) observations with the Owens Valley Millimeter Array constrain the mass of molecular gas to be less than 4x10^10 Mo, after correcting for lensing. The CO to sub-mm flux limit, the spectroscopic line ratios, and the spectral energy distribution for N4 are all within the range of properties found in other high-redshift sub-mm sources and local ultraluminous infrared galaxies. After the correction for lensing, N4 is the weakest intrinsic sub-mm selected source with a known redshift and represents the first redshift for the <2mJy 850um sources which are responsible for the bulk of the emission from the sub-mm population as a whole. We argue that N4 contains either an AGN or LINER nucleus surrounded by an extended region of active star-formation.

We describe a new wide field Hubble Space Telescope survey of the galaxy cluster Cl0024+16 (z~0.4) consisting of a sparse-sampled mosaic of 39 Wide Field and Planetary Camera 2 images which extends to a cluster radius of 5 Mpc. We examine both the morphology-radius (T-R) and morphology-density (T-Sigma) relations and demonstrate sensitivities adequate for measures from the core to a radius of 5 Mpc, spanning over 3 decades in local projected density. The fraction of early-type galaxies declines steeply from the cluster center to 1 Mpc radius and more gradually thereafter, asymptoting towards the field value at the periphery. We discuss our results in the context of three distinct cluster zones, defined according to different physical processes that may be effective in transforming galaxy morphology in each. By treating infalling galaxies as isolated test particles, we deduce that the most likely processes responsible for the mild gradient in the morphological mix outside the virial radius are harassment and starvation. Although more data are needed to pin down the exact mechanisms, starvation seems more promising in that it would naturally explain the stellar and dynamical homogeneity of cluster E/S0s. However, we find significant scatter in the local density at any given radius outside 0.5 Mpc, and that the same T-Sigma relation holds in subregions of the cluster, independent of location. In this hitherto unprobed region, where the potential of the cluster is weak, galaxies apparently retain their identities as members of infalling sub-groups whose characteristic morphological properties remain intact. Only upon arrival in the central regions is the substructure erased, as indicated by the tight correlation between cluster radius and Sigma.

We present optical and near-IR spectroscopy of a z=2.38 hyperluminous IR galaxy, covering the restframe wavelength range from 1000-5000A. It appears to comprise two components separated by less than 1" on the sky (<8kpc); one component (B) is blue, the other (P) is red in restframe UV-optical colours. The combined system has an optical luminosity of ~8L* and its restframe optical spectrum is characteristic of a Seyfert AGN. However, its restframe UV spectrum exhibits striking features associated with young stars, including P-Cygni lines from stellar winds and blue-shifted interstellar absorption lines indicative of a galactic outflow. Redshifts are derived from stellar photospheric lines in the UV and from narrow emission lines in the restframe optical, and these are compared to that measured for the molecular gas recently detected with the IRAM interferometer. The offsets indicate that the far-IR emission is most likely associated with the near-IR source P, which hosts the Seyfert nucleus, while the UV-bright component B is blueshifted by 400km/s. This suggests that the two components are probably merging and the resulting gravitational interactions have triggered the hyperluminous activity. Modelling of the UV spectral features implies that the starburst within the UV component of this system has been going on for at least ~10Myrs. Assuming that the bolometrically-dominant obscured component has a similar lifetime, we estimate that it has so far formed a total stellar mass of ~10^11 Mo. If this star formation continues at its present level for substantially longer, or if this activity is repeated, then the present-day descendant of N2 850.4 will be a very luminous galaxy.

We report on the X-ray, optical, near-infrared, submillimetre and radio properties of five Extremely Red Objects (EROs) selected at X-ray wavelengths by XMM-Newton in the Lockman Hole field. They all have enough counts in the X-ray band to allow spectral fitting: four are most probably obscured, Compton-thin AGN with redshift dependent absorbing column densities of 10^22 - 10^24/cm^2, whilst the fifth is best fitted by a thermal spectrum and is likely to be a massive elliptical galaxy in a deep gravitational potential. Their optical/near-infrared colours and sizes suggest that X-ray selected EROs comprise a mixture of dusty 'starburst' galaxies and non-dusty galaxies that are dominated by either star-light or light from an active nucleus. The colour diagnostics are supported by the submillimetre and radio data: the two AGN with 'starburst' colours have submillimetre or radio flux densities that imply large star-formation rates, whilst those with 'elliptical' colours do not. The one source detected in the submillimetre waveband has narrow emission lines at a redshift of 1.45. Although the bulk of its radio emission originates from processes other than star formation, it is most probably a radio-quiet ultraluminous infrared galaxy.

We use gravitational lens model and X-ray spectral analysis of ten X-ray luminous clusters at z~0.2 to study the impact of cluster substructure on attempts to normalize the matter power spectrum. We estimate that unrelaxed clusters are 30% hotter than relaxed clusters causing sigma-8 to be over-estimated by 20% if the cluster selection function is not correctly accounted for. This helps to explain the wide range in sigma-8 derived from different techniques, sigma-8~0.6-1, and offers a physically motivated explanation for some of this discrepency. We also identify two further systematics in our analysis: (i) extrapolation of small field-of-view mass measurements to the cluster virial radius and (ii) projection of 3-dimensional masses contained in numerical simulations to the 2-dimensional information that is available from observations. We combine quantitative estimates of these two effects with our model fitting to estimate from the current data that sigma-8=0.75 +/-0.05(statistical) +/-0.15 (systematic), where the systematic error reflects the extrapolation and projection uncertainties. All three systematics (substructure, extrapolation and projection) are fundanmental to future cluster-based measurements of sigma-8 regardless of the techniques employed. However, we identify gravitational lesning as the tool of choice for such studies, because a combination of strong and weak lensing offers the most direct route to control the systematics and thus achieve an unbiased comparison between observation and theory.

We analyse an 850-um SCUBA map of the environment of the z=2.39 radio galaxy 53W002, which has been shown to reside in an over-density of Ly-alpha detected galaxies. We identify four luminous submillimeter (submm) sources within a 2.3' (1.2Mpc at z=2.39) diameter area around the radio galaxy (which itself is a weak submm source). We employ a 1.4-GHz map to accurately locate the counterpart of one of these sources, SMMJ17142+5016, and identify this source with a narrow-line AGN with an extended Ly-alpha halo at z=2.390 which is member of the structure around 53W002. Hence SMMJ17142+5016 is the first spectroscopically-confirmed, submm-selected companion to a high-redshift radio galaxy. We discuss the OHS JHK spectrum of this galaxy and in addition present five new constraints on its spectral energy distribution long-ward of 1um, using these to estimate its bolometric luminosity as ~8x10^12 Lo, or a star formation rate of ~1000Mo/yr if young stars provide the bulk of the luminosity. This result provides direct support for the statistical detection of over-densities of SCUBA galaxies around high-redshift radio galaxies and confirms theoretical predictions that SCUBA galaxies, as the progenitors of massive ellipticals, should be strongly clustered in the highest density regions of the distant Universe.

We present the results from a sensitive multi-wavelength analysis of the properties of Extremely Red Objects (EROs). Our analysis employs deep RIzJHK photometry of a 8.5'x8.5' region to select a sample of 68 EROs with (R-K)>5.3 and brighter than K=20.5 (5-sigma). We combine this photometric dataset with an extremely deep 1.4-GHz radio map of the field obtained from the VLA. This map reaches a 1-sigma limiting flux density of 3.5uJy making it the deepest 1.4-GHz map taken and is sensitive enough to detect an active galaxy with L_1.4 > 10^23 W/Hz at z>1. If powered by a starburst, this radio luminosity is equivalent to a star-formation rate of >25Mo/yr for stars more massive than 5Mo. We identify radio counterparts to 21 of the EROs in this field with radio fluxes above 12.6uJy and resolve a third of these with our 1.6'' FWHM beam. The spectral energy distributions of the majority of these galaxies are consistent with those expected for dust-reddened starbursts at z~1. At these redshifts the radio luminosities of these galaxies indicate a median far-infrared luminosity of this population of L_FIR>10^12 Lo, meaning half of the sample are ultraluminous infrared galaxies (ULIRGs). We conclude that >16+/-5% of the ERO population brighter than K=20.5 are luminous infrared galaxies (LIRGs) at z>1. We also use photometric classification of the colors of the EROs to investigate the mix of dusty active and evolved passive systems in the remaining ERO population which is undetected in our radio map. Based on this we suggest that at least 30% and possibly up to ~60% of all EROs with (R-K)>5.3 and K<20.5 are dusty, star-forming systems at z>1. Our best estimate of the star formation density in this highly-obscured and optically faint (R>26) population is rho^* (0.1-100Mo)=0.11+/-0.03 Mo/yr/Mpc^3, comparable to estimates of that in H-alpha emitting galaxies at z~1, and greater than the estimates from UV-selected samples at these epochs. This lends support to the claims of a strong increase in the contribution from obscured systems to the star formation density at high redshifts. Using the observed counts of the radio-detected ERO population we model the apparent break in the K-band number counts of the whole ERO population at K~19-20 and propose that the passive ERO population dominates the total population in a relatively narrow magnitude range around K<20, with dusty, active EROs making up the bulk of the population at fainter limits.

We have identified counterparts to two submillimeter (submm) sources, SMMJ09429+4659 and SMMJ09431+4700, seen through the core of the z=0.41 cluster A851. We employ deep 1.4-GHz observations and the far-infrared/radio correlation to refine the submm positions and then optical and near-infrared imaging to locate their counterparts. We identify an extremely red counterpart to SMM\,J09429+4659, while GMOS spectroscopy with Gemini-North shows that the R=23.8 radio source identified with SMMJ09431+4700 is a hyperluminous infrared galaxy (LFIR~1.5x10^13Lo) at z=3.35, the highest spectroscopic redshift so far for a galaxy discovered in the submm. The emission line properties of this galaxy are characteristic of a narrow-line Seyfert-1, although the lack of detected X-ray emission in a deep XMM-Newton observation suggests that the bulk of the luminosity of this galaxy is derived from massive star formation. We suggest that active nuclei, and the outflows they engender, may be an important part of the evolution of the brightest submm galaxies at high redshifts.

We present the first spectroscopic survey of intrinsically low Lx clusters at z>>0, with HST WFPC2 imaging and ground-based spectroscopy. We study 172 confirmed cluster members in a sample of ten clusters at z=0.23-0.3, with Lx<4.E43 h^-2 ergs/s. The core of each cluster is imaged with WFPC2 in the F702W filter, and the spectroscopic sample is statistically complete to Mr=19.0+5Log(h), within an 11' field. The clusters are dynamically well-separated from the surrounding field and the velocity dispersions range from ~350-850 km/s. Emission line galaxies in these clusters are relatively rare, comprising only 22 +/-4% of the sample. There is no evidence that these emission-line galaxies are dynamically distinct from the majority of the cluster population, though our sample is too small to rule out the ~30% difference that has been observed in more massive clusters. We find eleven galaxies, comprising 6% of the cluster members, which are disk-dominated but show no sign of emission in their spectrum. Most of these are relatively isolated, spiral galaxies with smooth disks. We find no cluster members with a starburst or post-starburst spectrum. The striking similarity between the spectral and morphological properties of galaxies in these clusters and those of galaxies in more massive systems at similar redshifts implies that the physical processes responsible for truncating star formation in galaxies are not restricted to the rare, rich cluster environment, but are viable in much more common environments. In particular, we conclude that ram pressure stripping or cluster-induced starbursts cannot be solely responsible for the low star formation rates in these systems.

The detailed distribution of mass within clusters of galaxies can be used to pro be the nature of dark matter. We show that constraints on the extent of the mass distribution around galaxies in the rich cluster Abell 2218 obtained from combining strong and weak lensing ob servations are consistent with the predictions which assume that the dominant ma ss component (dark matter) in these halos is collisionless. A strongly interacting (fluid-like) dark matter is ruled-out at a confidence level of more than 5-sigma.

In this letter, we present the results of our study of galaxy-galaxy lensing in m assive cluster-lenses spanning z=0.17 to 0.58, utilizing high-quality archival Hubble Space Telescope (HST) data. Local anisotropies in the shear maps are assumed to arise from dark matter substructure within these clusters. Associating the substructure wi th bright early-type cluster galaxies, we quantify the properties of typical L* cluster members in a statistical fashion. The fraction of total mass associated with individual galaxies within the inner regions of these clusters ranges from 10-20% implying that the bulk of the dark matter in massive lensing clust ers is smoothly distributed. Looking at the properties of the cluster galaxies, we find strong evidence (>3-sigma significance) that a fiducial early-type L* galaxy in these clusters has a mass distribution that is tidally truncated compared to equivalent lumino sity galaxies in the field. In fact, we exclude field galaxy scale dark halos for these cluster early-types at >10-sigma significance. We compare the tidal radii obtained from this lensing analysis with the central density of the cluster potentials and find a correlation which is in excellent agreement with the oretical expectations of tidal truncation: Log r_t^* = (-0.6+/-0.2) Log rho_0.

The SCUBA 8-mJy survey is the largest submm extragalactic mapping survey underta ken to date, centred on the Lockman Hole and ELAIS N2 regions. Here, we present new 1.4-GHz imaging of the depth and resolution necessary to reliably identify radio counterparts for 18 of 30 submm sources. Armed with this greatly improved positional informa tion, we present and analyse new optical, near-IR and XMM-Newton X-ray imaging to identify optical/IR host galaxies to half of the submm-selected sources in those fields. As many as 15% of the sub mm sources detected at 1.4 GHz are resolved by the 1.4" beam and a further 25% have more than one radio coun terpart, suggesting that radio and submm emission arise from extended starbursts and that interactions are common. We note that less than a quarter of the submm-selected sample would have b een recovered by targeting optically faint radio sources, underlining the select ive nature of such surveys. At least 60% of the radio-confirmed optical/IR host galaxies appear to be morpholog ically distorted; many are composite systems - red galaxies with relatively blue companions; just over half are found to be very red or extremely red; contrary to popular belief, most are suffici ently bright to be tackled with spectrographs on 8-m telescopes. We estimate the median redshift of the S(850um)~8 mJy submm galaxy population: if the radio/far-IR correlation holds at high redshift, and our sample is unbiased, we derive a conservative median z >= 2.0, or >= 2.4 using spectral templates more representative of known submm galaxies.

Despite extensive observational efforts, the brightest sub--mm source in the Hubble Deep Field, HDF850.1, has failed to yield a convincing optical/infrared identification almost 4 years after its discovery. This failure is all the more notable given the availability of supporting multi-frequency data of unparalleled depth, and sub-arcsec positional accuracy for the sub-mm/mm source. Consequently, HDF850.1 has become a test case of the possibility that the most violently star-forming objects in the universe are too red and/or distant to be seen in the deepest optical images. Here we report the discovery of the host galaxy of HDF850.1. This object has been revealed by careful analysis of a new, deep K-prime image of the HDF obtained with the Subaru 8.2-m telescope. Its reality is confirmed by a similar analysis of the HST NICMOS F160W image of the same region. This object is extremely faint (K=23.5), clumpy (on sub-arcsec scales) and very red (I-K > 5.2; H-K = 1.4+/-0.35). The likelihood that it is the correct identification is strongly reinforced by a reanalysis of the combined MERLIN+VLA 1.4-GHz map of the field which yields a new radio detection of HDF850.1 only 0.1 arcsec from the new near-ir counterpart, and with sufficient positional accuracy to exclude all previously considered alternative optical candidates. We have calculated new confidence limits on the estimated redshift of HDF850.1 and find z = 4.1+/-0.5. We also calculate that the flux density of HDF850.1 has been boosted by a factor of ~3 through lensing by the intervening elliptical 3-586.0, consistent with predictions that a small but significant fraction of blank-field sub-mm sources are lensed by foreground galaxies. We discuss the wider implications of these results for the sub-mm population and cosmic star-formation history.

Spectroscopic surveys of luminous submillimetre-selected sources have uncovered optically-bright galaxies at z<1 close to the positions of several submillimetre (submm) sources. Naive statistical analyses suggest that these galaxies are associated with the submm emission. However, in some cases, it is difficult to understand this association given the relatively modest redshifts and unpreposessing spectral characteristics of the galaxies. These are in stark constrast to those expected from the massive dust-enshrouded starbursts and AGN thought to power the bulk of the bright submm population. We present new observations of optically-bright counterparts to two luminous submm sources, along with a compilation of previously proposed optically-bright counterparts with z<1. We suggest that the majority of these associations between bright galaxies and submm sources may be due to the action of the foreground galaxies as gravitational lenses on the much fainter and more distant submm sources. We discuss the implications of this conclusion for our understanding of the SCUBA population.

We present results of an H-alpha survey in the rich cluster A1689 at z=0.18, using the LDSS++ spectrograph on the AAT. We obtained spectra covering redshifted H-alpha for 522 galaxies brighter than I=22.5, covering a field of 8.7'x8.7'. We detect H-alpha emission in 46 of these galaxies; accounting for selection effects due to sampling and cluster membership, we determine that 24% of cluster members brighter than M_R=-16.5 + 5 Log(h) are detected with H-alpha flux greater than 4h^-2 x 10^38 ergs/s. This corresponds to a limiting star formation rate of 0.008 h^-2 Msun/yr, assuming 1 magnitude of dust extinction. From a Hubble Space Telescope mosaic covering 7.5' x 10.0', we determine morphologies for 199 galaxies brighter than I=21, and find that 20% of the cluster members are of type Sa or later. More than 90% of cluster spirals show H-alpha emission, compared with less than 10% of E and S0 galaxies. The cluster H-alpha luminosity function has a low normalisation relative to the z~0.2 field, by ~50%, after accounting for the different fraction of spiral galaxies in the two environments. When compared with local field galaxies, this suggests that star formation activity is suppressed in early-type cluster galaxies, relative to their field counterparts. Our sample includes 29 galaxies previously observed with ISOCAM at 6 and 15 microns. We detect all 15-micron sources at H-alpha, so there is no evidence for any star formation completely hidden at H-alpha. Comparing the 15-micron and H-alpha fluxes, we find evidence that some mid-infrared-detected galaxies could be obscured by as much as 3 magnitudes of extinction at H-alpha, although this depends on the largely unknown contribution from any AGN-heated dust to the mid-infrared flux.

We present near-infrared spectroscopy and Hubble Space Telescope (HST) imaging of EROJ003707+0909.5, the brightest of three gravitationally-lensed images of an Extremely Red Object (ERO) at z=1.6, in the field of the massive cluster A68 (z=0.255). We exploit the superlative resolution of our HST data and the enhanced spatial resolution and sensitivity afforded by the lens amplification to reconstruct the source-plane properties of this ERO. Our morphological and photometric analysis reveals that EROJ003707 is an L* early-type disk-galaxy and we estimate that ~10 per cent of EROs with (R-K)>=5.3 and K<=21 may have similar properties. The unique association of passive EROs with elliptical galaxies therefore appears to be too simplistic. We speculate on the evolution of EROJ003707: if gas continues to cool onto this galaxy in the manner predicted by hierarchical galaxy formation models, then by the present day, EROJ003707 could evolve into a very luminous spiral galaxy.

We present Keck spectroscopy and UKIRT near-IR imaging observations of two 170um-selected sources from the ISO-FIRBACK survey which have faint counterparts in the optical, and r-K~5. Both sources were expected to lie at z>1 based on their far-infrared, submillimeter and radio fluxes, assuming a similar spectral energy distribution to the local ultra-luminous infrared galaxy (ULIRG) Arp220. However, our spectroscopy indicates that the redshifts of these galaxies are z<1: z=0.91 for FN1-64 and z=0.45 for FN1-40. While the bolometric luminosities of both galaxies are similar to Arp220, it appears that the dust emission in these systems has a characteristic temperature of 30K much cooler than the ~50K seen in Arp220. Neither optical spectrum shows evidence of AGN activity. If these galaxies are characteristic of the optically faint FIRBACK population, then evolutionary models of the far-infrared background must include a substantial population of cold, luminous galaxies. These galaxies provide an important intermediate comparison between the local luminous IR galaxies, and the high redshift submillimeter-selected galaxies, for which there is very little information available.

A cosmologically significant population of very luminous high-redshift galaxies has recently been discovered at submillimeter (submm) wavelengths. Advances in submm detector technologies have opened this new window on the distant Universe. Here we discuss the properties of the high-redshift submm galaxies, their significance for our understanding of the process of galaxy formation, and the selection effects that apply to deep submm surveys. The submm galaxies generate a significant fraction of the energy output of all the galaxies in the early Universe. We emphasize the importance of studying a complete sample of submm galaxies, and stress that because they are typically very faint in other wavebands, these follow-up observations are very challenging. Finally, we discuss the surveys that will be made using the next generation of submm-wave instruments under development.

We present the source catalogue for the SCUBA Lens Survey. We summarise the results of extensive multi-wavelength observations of the 15 submillimetre-selected galaxies in the catalogue, from X-rays to radio. We discuss the main observational characteristics of faint submillimetre galaxies as a population, and consider their interpretation within the framework of our understanding of galaxy formation and evolution.

We present a photometric investigation of the variation in galaxy colour with environment in 11 X-ray luminous clusters at 0.07<=z<=0.16 taken from the Las Campanas/AAT Rich Cluster Survey. We study the properties of the galaxy populations in individual clusters and take advantage of the homogeneity of the sample to combine the clusters together to investigate weaker trends in the composite sample. We find that modal colours of galaxies lying on the colour-magnitude relation in the clusters become bluer by d(B-R)/d(r_p) = -0.022+/-0.004 from the cluster core out to a projected radius of r_p = 6Mpc; further out in radius than any previous study. We also examine the variation in modal galaxy colour with local galaxy density, Sigma, for galaxies lying close to the colour-magnitude relation and find that the median colour shifts bluewards by d(B-R)/dLog(Sigma) = -0.076+/-0.009 with decreasing local density across three orders of magnitude. We show that the position of the red envelope of galaxies in the colour-magnitude relation does not vary as a function of projected radius or density within the clusters, suggesting that the change in the modal colour results from an increasing fraction of bluer galaxies within the colour-magnitude relation, rather than a change in the colours of the whole population. We show that this shift in the colour-magnitude relations with projected radius and local-density is greater than expected from the changing morphological mix based on the local morphology-density relation. We therefore conclude that we are seeing a real change in the properties of galaxies on the colour-magnitude relation in the outskirts of clusters. The simplest interpretation of this result (and similar constraints in local clusters) is that an increasing fraction of galaxies in the lower density regions at large radii within clusters exhibit signatures of star formation in the recent past, signatures which are not seen in the evolved galaxies in the highest density regions.

We present deep, panoramic multi-color imaging of the distant rich cluster A851 (Cl0939+4713, z=0.41) using Suprime-Cam on Subaru. These images cover a 27' field of view, ~11 Mpc at z=0.41, and by exploiting photometric redshifts estimated from our BVRI imaging we can isolate galaxies in a narrow redshift slice at the cluster redshift. Using a sample of ~2700 probable cluster members brighter than 0.02 Lv*, we trace the network of filaments and subclumps around the cluster core. The depth of our observations, combined with the identification of filamentary structure, gives us an unprecedented opportunity to test the influence of the environment on the properties of low luminosity galaxies. We find an abrupt change in the colors of < 0.1 Lv* galaxies at a local density of 100 gal. per sq. Mpc, with the population in lower density regions being predominantly blue, while those in higher density regions are red. The transition in the color-local density behavior occurs at densities corresponding to subclumps within the filaments surrounding the cluster. Identifying the sites where the transition occurs brings us much closer to understanding the mechanisms which are responsible for establishing the present-day relationship between environment and galaxy characteristics.

We present a morphological analysis of 17 X-ray selected clusters at z~0.25, imaged uniformly with Hubble Space Telescope WFPC2. Eight of these clusters comprise a subsample selected for their low X-ray luminosities (<10^44 erg/s), called the Low-Lx sample. The remaining nine clusters comprise a High-Lx subsample with Lx>10^45 ergs/s. The two subsamples differ in their mean X-ray luminosity by a factor of 30, and span a range of more than 300. The clusters cover a relatively small range in redshift (z=0.17-0.3, dz/z~0.15) and the data are homogeneous in terms of depth, resolution (0.17"=1 kpc at z=0.25) and rest wavelength observed, minimizing differential corrections from cluster to cluster. We fit the two dimensional surface brightness profiles of galaxies down to very faint absolute magnitudes: Mag<-18.2+5 logh_50 (roughly 0.01 L*_R) with parametric models, and quantify their morphologies using the fractional bulge luminosity (B/T). Within a single WFPC2 image, covering a field of ~3 arcmin (1 Mpc at z=0.25) in the cluster centre, we find that the Low-Lx clusters are dominated by galaxies with low B/T (~0), while the High-Lx clusters are dominated by galaxies with intermediate B/T (~0.4). We test whether this difference could arise from a universal morphology-density relation due to differences in the typical galaxy densities in the two samples. We find that small differences in the B/T distributions of the two samples persist with marginal statistical significance (98% confidence based on a binned chi^2 test) even when we restrict the comparison to galaxies in environments with similar projected local galaxy densities. A related difference (also of low statistical significance) is seen between the bulge luminosity functions of the two cluster samples, while no difference is seen between the disk luminosity functions. From the correlations between these quantities, we argue that the global environment affects the population of bulges, over and above trends seen with local density. On the basis of this result we conclude that the destruction of disks through ram pressure stripping or harassment is not solely responsible for the morphology-density relation, and that bulge formation is less efficient in low mass clusters, perhaps reflecting a less rich merger history.

We present new, multi-wavelength, high-resolution imaging of the luminous, submillimeter galaxy, SMMJ14011+0252, an interacting starburst at z=2.56. Our observations comprise optical imaging from the HST, sensitive radio mapping from the VLA and CO observations from OVRO and BIMA. Aided by well-constrained gravitational amplification, we use these new data to map the distribution of gas and both obscured and unobscured starlight. The maps show that the gas and star formation are extended on scales of >= 10 kpc, much larger than starbursts seen in local ultraluminous galaxies, and larger than the rest-frame UV-bright components of SMMJ14011+0252, J1/J2. The most vigorous star formation is marked by peaks in both the molecular gas and radio emission, ~1" north of J1/J2, in the vicinity of J1n, an apparent faint extension of J1. Using new sub-0.5" K-band imaging from UKIRT, we identify J1n as an extremely red object (ERO). We suggest that while J1 and J2 are clearly associated with the submm source, they are merely windows through the dust, or unobscured companions to a large and otherwise opaque star-forming system. Hence, their rest-frame UV properties are unlikely to be relevant for understanding the detailed internal physics of the starburst.

We present near-IR spectroscopy and HST imaging of EROJ164023, an Extremely Red Object (ERO) with R-K=5.9 at z=1.05. EROJ164023 is a disk galaxy, with an optical/IR spectral energy distribution which is strongly reddened by dust (L_FIR/L_B<~200; A_V~5). The narrow emission lines (~300km/s) and the high [NII]/Halpha line ratio indicate that this is a ``composite'' starburst-Seyfert galaxy. Assuming that star formation dominates, we constrain the SFR to be 10-700Mo/yr from a variety of indicators. We compare EROJ164023 with the only other spectroscopically identified dusty EROs: HR10 (z=1.44) and ISOJ1324-2016 (z=1.50). EROJ164023 and HR10 have similar disk-like morphologies, and both exhibit a variation in the apparent dust obscuration depending upon the diagnostic used, suggesting that there is a complex spatial mix of stellar populations and dust in these galaxies. In contrast, the compact morphology and spectral properties of ISOJ1324-2016 indicate that it is a dusty quasar. Our results demonstrate that dusty galaxies identified using photometric ERO criteria include pure starbursts, composite systems such as EROJ164023 and dusty quasars. We suggest that the classification of EROs into these sub-classes cannot be reliably achieved from optical/near-IR photometry and instead requires mid/far-IR or sub-mm photometry and near-IR spectroscopy. The advent of efficient multi-object spectrographs working in the near-IR as well as the imminent launch of SIRTF therefore promise the opportunity of rapid progress in our understanding of the elusive ERO population.

We discuss the possibility of using OH and H2O gigamasers to trace the redshift distribution of luminous, dust-obscured, star-forming galaxies. It has long been thought that ultraluminous, interacting galaxies should host gigamasers due to their vast pumping infrared luminosity, the large column density of molecules available to populate the maser states and the turbulent motion of the gas in these dynamically complex systems which allows unsaturated maser emission. OH masers may thus be well-suited to the redshift-blind detection of ultraluminous and hyperluminous infrared galaxies (L_FIR > 10^12 Lo) such as those uncovered by the SCUBA submillimetre camera. The bandwidth requirement is low, <1GHz for z=1-10 (lower still if additional redshift constraints are available) and the dual-line 1665-/1667-MHz OH spectral signature can act as a check on the reality of detections.

We present the first results of a spectroscopic survey of faint lensed galaxie s in the core of the galaxy cluster AC114 (z=0.312) obtained from observations w ith the FORS1 spectrograph mounted on the VLT-Antu. The galaxies were chose n in areas close to the high-z critical lines predicted by the gravitational lens model of Natarajan et al (NKSE, 1998) for this cluster, according to both lensing and photometric redshift criteria. All the target galaxies are found to correspond to background galaxies with redshifts values in the [0.7, 3.5] interval. Our spectroscopic observations confirm the predicted lensing redshifts for 3 of the multiple-image galaxies, and together with predictions of the NKSE model led to the discovery of a new 5-image configuration at redshift z=3.347. A revised NKSE model, compatible with the redshift of this new multiple-image system, was generated and employed to calculate the gravitational amplifications of all the observed galaxies. The galaxies corresponding to the multiple-image systems are found to be intrinsically fainter, between 0.5 and 1.5 magnitudes, than the limiting magnitudes of existing blank field studies. When all the observed background galaxies are considered, the resulting intrinsic absolute magnitudes range from M_B~-22 to -19. Therefore, a large gain in sensitivity towards low luminosity high-z objects can actually be obtained, in agreement with theoretical expectations. This method can be used advantageously to probe the high redshift Universe and, in particular, its application to an ensemble of massive cluster cores could constraint the faint end of luminosity function of high redshift galaxies.

We present the results of a survey for Extremely Red Objects (EROs) in the fields of ten galaxy cluster lenses at z~0.2, combining high-resolution Hubble Space Telescope imaging with deep K-band imaging from UKIRT. We detect 60 EROs with R-K>=5.3, of which 26 have R-K>=6.0 in a total image plane survey area of 49arcmin^2 down to K=20.6. We use detailed models of the cluster lenses to quantify the lens amplification and thus correct the observed number counts and survey area for the effects of gravitational lensing. After making these corrections, we estimate surface densities at K<=21.6 of 2.5+/-0.4 and 1.2+/-0.3 arcmin^-2 for EROs with R-K>=5.3 and 6.0 respectively. These ERO number counts agree with previous shallower surveys at K<~19 and flatten significantly at magnitudes fainter than K~19-20. This flattening may be due to a transition from an ERO population dominated by evolved galaxies at z~1-2 (K<~19.5) to one dominated by dusty starburst galaxies at z>1 (K>~19.5). We also compare our results with various model predictions, including a model that attempts to explain EROs in terms of a single population of elliptical galaxies formed at high redshift. We find that a formation epoch of z_f~2.5 for this population matches the observed surface density of R-K>=5.3 EROs quite well, and the R-K>=6.0 sample less well. More sophisticated models, including semi-analytic prescriptions, under-predict the ERO surface density by approximately an order of magnitude, suggesting that these models produce insufficient stars and/or dust at high redshift.

The Las Campanas Observatory and Anglo--Australian Telescope Rich Cluster Survey (LARCS) is a panoramic imaging and spectroscopic survey of an X-ray luminosity-selected sample of 21 clusters of galaxies at 0.07

We present high signal-to-noise, moderate-resolution spectroscopy of 48 early-type members of the rich cluster Abell 2218 at z=0.18 taken with the LDSS2 spectrograph on the 4.2-m William Herschel Telescope. This sample is both larger and spans a wider galaxy luminosity range, down to M*+1, than previous studies. In addition to the relatively large size of the sample we have detailed morphological information from archival Hubble Space Telescope imaging for 20 of the galaxies. We combine the morphological, photometric, kinematic and line-strength information to compare A2218 with similar samples drawn from local clusters and to identify evolutionary changes between the samples which have occured over the last ~3 Gyrs. The overall picture is one of little or no evolution in nearly all galaxy parameters. Zeropoint offsets in the Faber-Jackson, Mgb-sigma and Fundamental Plane relations are all consistent with passively evolving stellar populations. The slopes of these relations have not changed significantly in the 3 Gyrs between A2218 and today. We do however find a significant spread in the estimated luminosity-weighted ages of the stellar populations in the galaxies, based on line diagnostic diagrams. This age spread is seen in both the disky early-type galaxies (S0) and also the ellipticals. We observe both ellipticals with a strong contribution from a young stellar population and lenticulars dominated by old stellar populations. On average, we find no evidence for systematic differences between the populations of ellipticals and lenticulars. In both cases there appears to be little evidence for differences between the stellar populations of the two samples. This points to a common formation epoch for the bulk of the stars in most of the early-type galaxies in A2218. This result can be reconciled with the claims of rapid morphological evolution in distant clusters if the suggested transformation from spirals to lenticulars does not involve significant new star formation.

We present the discovery of the Optical Transient (OT) of the long-duration gamma-ray burst GRB000926. The optical transient was detected independently with the Nordic Optical Telescope and at Calar Alto 22.2 hours after the burst. At this time the magnitude of the transient was R = 19.36. The transient faded with a decay slope of about 1.7 during the first two days after which the slope increased abruptly (within a few hours) to about 2.4. The light-curve started to flatten off after about a week indicating the presence of an underlying extended object. This object was detected in a deep image obtained one month after the GRB at R=23.87+/-0.15 and consists of several compact knots within about 5 arcsec. One of the knots is spatially coincident with the position of the OT and hence most likely belongs to the host galaxy. Higher resolution imaging is needed to resolve whether all the compact knots belong to the host galaxy or to several independent objects. In a separate paper we present a discussion of the optical spectrum of the OT, and its inferred redshift (Moller et al. in prep.).

We present the results of a wide-field survey for H-alpha emitting galaxies in the cluster AC114 at z=0.32. Spectra centred on H-alpha at the cluster redshift have been obtained for 586 galaxies to I~22 out to a radius around 2Mpc. At most, only ~10% of these were found to be H-alpha-emitting cluster members. These objects are predominantly blue and of late-type spiral morphology, consistent with them hosting star formation. However, ~65% of the cluster members classified morphologically as spirals (with HST), have no detectable H-alpha emission; star-formation and morphological evolution in cluster galaxies appear to be largely decoupled. Changes in the H-alpha detection rate and the strength of H-alpha emission with environment are found to be weak within the region studied. Star formation within the cluster members is also found to be strongly and uniformly suppressed, with the rates inferred from the H-alpha emission not exceeding 4Mo/yr, and AC114's H-alpha luminosity function being an order of magnitude below that observed for field galaxies at the same redshift. None of the galaxies detected have the high star formation rates associated with `starburst' galaxies; however, this may still be reconciliable with the known (8+/-3%) fraction of `post-starburst' galaxies within AC114, given the poorly determined but short lifetimes of starbursts and the possibility that much of the associated star formation is obscured by dust.

We present an analysis of the mass distribution in the core of A383 (z=0.188), one of twelve X-ray luminous galaxy clusters at z~0.2 selected for a comprehensive and unbiased study of the mass distribution in massive galaxy clusters. Deep optical imaging performed by the HST reveals a wide variety of gravitationally lensed features in the core of A383, including a giant arc formed from the strongly-lensed images of two background galaxies, two radial arcs in the halo of the central cluster galaxy, several multiply-imaged arcs and numerous arclets. Based upon the constraints from the various lensed features, as well as from color information from ground-based observations, we construct a detailed model of the mass distribution in the central regions of the cluster, taking into account both the cluster-scale potential and perturbations from individual cluster galaxies. Keck spectroscopy of one component of the giant arc identifies it as an image of a star-forming galaxy at z=1.01 and provides an accurate measurement of the mass of the cluster within the projected radius of the giant arc (65kpc) of (3.5+/-0.1)*10^13 Mo. Using the weak shear measured from our HST observations we extend our mass model to larger scales and determine a mass of (1.8+/-0.2)*10^14 Mo within a radius of 250kpc. On smaller scales we employ the radial arcs as probes of the shape of the mass distribution in the cluster core (r<20kpc), find that the mass profile is more peaked than a single NFW profile. Our findings therefore support the proposal that massive cluster cores contain more mass than can be explained by a single cluster-scale NFW profile. The optical and X-ray properties of A383 indicate the presence of a central cooling flow, for which we derive a mass deposition rate of >200 Mo/yr. We also use the X-ray emission from A383 to obtain independent estimates of the total mass within projected radii of 65 and 250kpc: (4.0+/-1.4)*10^13 Mo and (1.2+/-0.5)*10^14 Mo, which are consistent with the lensing measurements.

Hubble Space Telescope observations of distant clusters have suggested a steep increase in the proportion of S0 galaxies between the distant clusters and clusters at the present-day. It has been proposed that this increase results from the transformation of the morphologies of accreted field galaxies from spirals to S0s. We have simulated the evolution of the morphological mix in clusters based on a simple model in order to test this morphological transformation hypothesis. In order to reproduce the apparently rapid increase in the ratio of S0 galaxies to ellipticals in the clusters, our model requires that: (1) galaxy accretion rate has to be high (more than half of the present-day cluster population must have been accreted since z~0.5), and (2) most of the accreted spirals, with morphological types as late as Scdm, must have transformed to S0's. Although the latter requirement may be difficult to meet, it is possible that such bulge-weak spirals have already been `pre-processed' into the bulge-strong galaxies prior to entering the cluster core and are eventually transformed into S0's in the cluster environment. On the basis of the evolution of the general morphological mix in clusters our model suggests that the process responsible for the morphological transformation takes a relatively long time (~2Gyr) after the galaxy has entered the cluster environment.

We present deep optical and near-infrared imaging of the rich cluster A2218 at z=0.17. Our optical imaging comes from new multicolour Hubble Space Telescope WFPC2 observations in the F450W (B), F606W (V) and F814W (I) passbands. These observations are complemented by deep near-infrared, Ks-band, imaging from the new INGRID imager on the 4.2-m William Herschel Telescope. This combination provides unique high-precision multicolour optical-infrared photometry and morphological information for a large sample of galaxies in the core of this rich cluster at a lookback time of ~3Gyrs. We analyse the (B-I), (V-I) and (I-Ks) colours of galaxies spanning a range of a factor of 100 in K-band luminosity in this region and compare these with grids of stellar population models. We find that the locus of the colours of the stellar populations in the luminous (>0.5L*) early-type galaxies, both ellipticals and S0s, traces a sequence of varying metallicity at a single age. At fainter luminosities (<0.1L*), this sequence is extended to lower metallicities by the morphologically-classified ellipticals. However, the faintest S0s exhibit very different behaviour, showing a wide range in colours, including a large fraction (30%) with relatively blue colours which appear to have younger luminosity-weighted ages for their stellar populations, 2-5Gyrs. We show that the proportion of these young S0s in the cluster population is consistent with the observed decrease in the S0 population seen in distant clusters when interpreted within the framework of a two-step spectroscopic and morphological transformation of accreted spiral field galaxies into cluster S0s.

We report the detection of 1.3mm continuum and near-infrared K-band (2.2-um) emission from the submillimeter galaxy SMM J00266+1708. Although this galaxy is among the brightest sub-mm sources detected in the blank-sky surveys (L~10^13 Lo), SMM J00266+1708 had no previously known optical/near-infrared counter-part. We used sensitive interferometric 1.3mm observations with the Owens Valley Millimeter Array to accurately determine the position of the sub-mm galaxy. Follow-up near-infrared imaging with the Keck I telescope uncovered a new faint red galaxy at K=22.5 mag which is spatially coincident with the 1.3mm emission. This is currently the faintest confirmed counter-part of a sub-mm galaxy. Although the redshift of SMM J00266+1708 is still unknown, its high sub-mm/radio spectral index suggests that the system is at high redshift (z>2). Approximately 50% or more of the sub-mm galaxies are faint/red galaxies similar to that of SMM J00266+1708. These ultraluminous obscured galaxies account for a significant fraction of the total amount of star-formation at high redshift despite being missed by optical/ultraviolet surveys.

Distant Type Ia and II supernovae (SNe) can serve as valuable probes of the history of the cosmic expansion and star formation, and provide important information on their progenitor models. At present, however, there are few observational constraints on the abundance of SNe at high redshifts. A major science driver for the Next Generation Space Telescope (NGST) is the study of such very distant supernovae (SNe). In this paper we discuss strategies for finding and counting distant SNe by using repeat imaging of super-critical intermediate redshift clusters whose mass distributions are well-constrained via modelling of strongly-lensed features. For a variety of different models for the star formation history and supernova progenitors, we estimate the likelihood of detecting lensed SNe as a function of their redshift. In the case of a survey conducted with HST, we predict a high probability of seeing a supernova in a single return visit with either WFPC-2 or ACS, and a much higher probability of detecting examples with z>1 in the lensed case. Most events would represent magnified SNe II at z~1, and a fraction will be more distant examples. We discuss various ways to classify such events using ground-based infrared photometry. We demonstrate an application of the method using the HST archival data and discuss the case of a possible event found in the rich cluster AC 114 (z=0.31).

Biased galaxy-formation theories predict that massive galaxies at high redshifts should act as signposts to high-density environments in the early Universe, which subsequently evolve into the cores of the richest clusters seen at the present day. These regions are characterised by over-densities of young galaxies, perhaps including a population of dusty, interaction-driven starbursts - the progenitors of massive cluster ellipticals. By searching for this population at submm wavelengths we can therefore test both galaxy- and structure-formation models. We have undertaken such a search in the field of a z=3.8 radio galaxy, 4C41.17, with the SCUBA submm camera. Our extremely deep 450- and 850-micron maps reveal an order-of-magnitude over-density of luminous submm galaxies compared to typical fields (the likelihood of finding such an over-density in a random field is <2E-3). The SCUBA galaxies have bolometric luminosities, >10^13 L(solar), which imply star-formation rates consistent with those required to form a massive galaxy in only a few 10^8 years. We also note that this field exhibits an over-density of extremely red objects (EROs), some of which may be associated with the submm sources, and Lyman-break galaxies. We propose that the over-densities of both submm and ERO sources in this field represent young dusty, starburst galaxies forming within a proto-cluster centered on the radio galaxy at z=3.8, which is also traced by a less-obscured population of Lyman-break galaxies.

The powerful combination of the Chandra X-ray telescope, the SCUBA submillimetre-wave camera and the gravitational lensing effect of the massive galaxy clusters A2390 and A1835 has been used to place stringent X-ray flux limits on six faint submillimetre sources found with SCUBA and deep submillimetre limits on three Chandra sources which all lie in fields common to both instruments. A single further source is marginally detected in both the X-ray and submillimetre bands. For the SCUBA-detected sources our results are consistent with starburst-dominated emission. The objects for which the strongest constraints can be placed, which include SMMJ14011+0252 at z=2.55, can only host powerful active galactic nuclei if they are both Compton-thick and any scattered X-ray flux is weak or itself absorbed. The lensing amplification for the sources are in the range 1.5-7, assuming that they lie at z>1. The brightest detected X-ray source has a faint extended optical counterpart (I~22) with colours consistent with a galaxy at z~1. The X-ray spectrum of this object is hard, implying strong intrinsic absorption with a column density of about 10^23 cm^-2 and an intrinsic (unabsorbed) 2-10keV luminosity of 3x10^44 ergs/s. This object is therefore a Type II quasar. The weakest detected X-ray sources are not detected in HST imaging of the cluster down to I~26.

Near infrared imaging and spectroscopic observations of the extremely red object (R-K~ 7 mag) CL 0939+4713 B have been obtained with the Near Infrared Camera on the Keck I Telescope of the W. M. Keck Observatory. The imaging shows a slightly elongated structure, while the spectroscopy shows a continuum break that allows us to determine the redshift of z = 1.58 + 0.01/-0.03 for this system. The fits of a range of models to the infrared spectrum suggests that it is predominantly an old (> 10^9 yrs) stellar system that suffers little extinction, while the measurerd R and I magnitudes suggests an age of ~ 3 x 10^8 years. The limit on the equivalent width of any emission line in the infrared spectrum argues that CL 0939+4713 B is not an actively star forming galaxy. This system, though similar in R-K color to HR 10 [also known as J1645+46] (Dey et al. 1999), is much different in morphology and emission line strengths, demonstrating the heterogeneity of extremely red extragalactic objects (EROs) selected on the basis of large values of R-K.

We present deep radio maps from the Very Large Array (VLA) for 16 sources detected in a sub-millimeter (submm) survey of the distant Universe. Our deep VLA 1.4-GHz maps allow us to identify radio counterparts or place stringent limits (<20uJy in the source plane) on the radio flux of the submm sources. We compare the spectral indices of our sources between 850um and 1.4GHz to empirical and theoretical models for distant starburst galaxies and active galactic nuclei (AGN) as a function of redshift. In this way we can derive redshift limits for the submm sources, even in the absence of an optical or near-infrared counterpart. We conclude that the submm population brighter than ~1mJy has a median redshift of at least ~2, more probably ~2.5-3, with almost all galaxies at z>>1. This estimate is a strong lower limit as both misidentification of the radio counterparts and non-thermal emission from an AGN will bias our redshift estimates to lower values. The high median redshift means that the submm population, if predominately powered by starbursts, contributes a substantial fraction of the total star formation density at high redshifts. A comparison of the spectral index limits with spectroscopic redshifts for proposed optical counterparts to individual submm galaxies suggests that half of the submm sources remain unidentified and thus their counterparts must be fainter than I>24.

We present extensive observations of a sub-sample of distant, submillimetre (submm) galaxies detected in the field of the massive cluster lens, Abell 1835, using the Submm Common-User Bolometer Array (SCUBA). Taken in conjunction with earlier observations of other submm-selected sources (Ivison et al. 1998; Smail et al. 1999; Soucail et al. 1999) we now have detailed, multi-wavelength observations of seven examples of the submm population, having exploited the combination of achromatic amplification by cluster lenses and lavish archival datasets. These sources, all clearly at z>1, illustrate the wide range in the properties and nature of distant submm-selected galaxies. We include detailed observations of the first candidate `pure' starburst submm galaxy at high redshift, a z=2.56 interacting galaxy which shows no obvious sign of hosting an active galactic nucleus (AGN). The remaining sources in this field have varying degrees of inferred AGN activity, although even when an AGN is obviously present it is still not apparent if reprocessed radiation from this source dominates the submm emission from the galaxy. In contrast with the variation in the spectral properties, we see relatively homogeneous morphologies for the population with a large fraction of merging or interacting systems. A picture of the submm galaxy population is emerging where virtually identical spectral energy distributions (SEDs) are seen for galaxies with very different optical/UV spectral characteristics, indicating that classification of these galaxies on the basis of easily obscured rest-frame optical/UV line emission may be misleading and that we must seek other means of determining the various contributions to the overall energy budget.

A large fraction of the luminous distant submm-wave galaxies recently detected using SCUBA on the JCMT appear to be associated with interacting optical counterparts. We investigate these systems using a simple hierarchical clustering model of galaxy evolution, in which the large luminosity of the SCUBA galaxies is assumed to be generated at the epoch of galaxy mergers in a burst of either star formation activity or the fueling of an active galactic nucleus (AGN). The models are well constrained by the observed spectrum of the FIR/submm background radiation and the 60-micron counts of low-z IRAS galaxies. The ratio between the total amount of energy released during a merger and the mass of dark matter involved must increase sharply with z at z<1, and then decrease at higher z, independent of the fraction of the luminosity of mergers that is produced by starbursts and AGN. One additional parameter - the reciprocal of the product of the duration of the enhanced luminosity produced by the merger and the fraction of mergers that induce an enhanced luminosity, which we call the activity parameter - is introduced, to allow the relationship between merging dark matter haloes and the observed counts of distant dusty galaxies to be investigated. The observed counts can only be reproduced if the activity parameter is greater by a factor of about 10 and 200 at redshifts of 1 and 3 respectively as compared with the present epoch. Hence, if mergers account for the SCUBA galaxies, then the merger process must have been much more violent at high redshifts. We discuss the counts of galaxies and the intensity of background radiation in the optical/near-IR wavebands in the context of these hierarchical models, and thus investigate the relationship between the populations of SCUBA and Lyman-break galaxies.

We present deep Hubble Space Telescope NICMOS near-infrared and WFPC2 optical imaging of a small region in the core of the distant rich cluster Cl0939+4713 (z=0.41). We compare the optical and near-infrared morphologies of cluster members and find apparent small-scale optical structures within the galaxies which are absent in the near-infrared. We conclude that strong dust obscuration is a common feature in the late-type galaxies in distant clusters. We then concentrate on a sample of ten faint radio galaxies lying within our NICMOS field and selected from a very deep 1.4-GHz VLA map of the cluster with a 1sigma flux limit of 9uJy. Using published data we focus on the spectral properties of the eight radio-selected cluster members and show that these comprise a large fraction of the post-starburst population in the cluster. The simplest interpretation of the radio emission from these galaxies is that they are currently forming massive stars, contradicting their classification as post-starburst systems based on the optical spectra. We suggest that this star formation is hidden from view in the optical by the same obscuring dust which is apparent in our comparison on the optical and near-infrared morphologies of these galaxies. We caution that even in the restframe optical the effects of dust cannot be ignored when comparing samples of distant galaxies to low-redshift systems, particularly if dust is as prevelant in distant galaxies as appears to be the case in our study.

We combine morphological classifications from deep HST imaging of a sample of 3 clusters at z=0.31 and a further 9 clusters at z=0.37-0.56 with existing spectroscopic observations of their E and S0 populations, to study the relative spectral properties of these two galaxy types. We have also used similar data for the Coma cluster as a present day bench-mark with which to compare our data at higher redshift. The data span the range where strong evolution is claimed in the proportion of S0 galaxies within rich clusters. Techniques have been recently developed to analyse the strengths of absorption lines in the spectra of local, passive galaxies to separate the effects of age & metallicity and hence date the ages of the most recent, substantial star formation episode in these galaxies. We have applied these techniques to the co-added spectra of the distant cluster galaxies to determine the relative ages of the E and S0 populations. We find no statistically significant difference between the luminosity-weighted ages of the E and S0 galaxies in these clusters. The precision of this measurement would indicate that any recent star formation in the cluster S0s at z=0.31 could have involved bursts of no more than ~11% (by mass) in strength in no more than half of these galaxies. Our results, in conjunction with other work, suggest that the progenitors of S0 galaxies in rich clusters are mostly early-type spirals who, through interactions with the cluster environment, have had their star formation truncated. This indicates a relatively unspectacular origin for the missing S0 population at high redshift.

We have used deep ground-based imaging in the near-infrared to search for counterparts to the luminous submillimeter (submm) sources in the catalog of Smail et al (1998). For the majority of the submm sources the near-IR imaging supports the counterparts originally selected from deep optical images. However, in two cases (10% of the sample) we find a relatively bright near-IR source close to the submm position, sources that were unidentified in the deep HST and ground-based R-band images used in Smail et al (1998). We place limits on colours of these sources from deep high-resolution Keck II imaging and find they have 2-sigma limits of (I-K)>6.8 and (I-K)>6.0 respectively. Both sources thus class as extremely red objects (EROs). Using the spectral properties of the submm source in the radio and submm we argue that these EROs are probably the source of the submm emission, rather than the bright spiral galaxies previously identified by Smail et al. (1998). This connection provides important insights into the nature of the enigmatic ERO population and faint submm galaxies in general. From the estimated surface density of these submm-bright EROs we suggest that this class accounts for the majority of the reddest members of the ERO population, in good agreement with the preliminary conclusions of pointed submm observations of individual EROs. We conclude that the most extreme EROs represent a population of dusty, ultraluminous galaxies at high redshifts; further study of these will provide useful insights into the nature of star formation in obscured galaxies in the early Universe. The identification of similar counterparts in blank field submm surveys will be extremely difficult owing to their faintness (K~20.5, I>26.5). Finally, we discuss the radio and submm properties of the two submm-bright EROs discovered here and suggest that both galaxies lie at z>2.

We report on X-ray, optical and infrared follow-up observations of GRB 980703. We detect a previously unknown X-ray source in the GRB error box; assuming a power law decline we find for its decay index alpha < -0.91 (3-sigma). We invoke host galaxy extinction to match the observed spectral slope with the slope expected from `fireball' models. We find no evidence for a spectral break in the infrared to X-ray spectral range on 1998 July 4.4, and determine a lower limit of the cooling break frequency: nu_c > 1.3 x 10^17 Hz. For this epoch we obtain an extinction of A_V = 1.50 +/- 0.11. From the X-ray data we estimate the optical extinction to be A_V = 20.2 +12.3 -7.3, inconsistent with the former value. Our optical spectra confirm the redshift of z = 0.966 found by Djorgovski et al. (1998). We compare the afterglow of GRB 980703 with that of GRB 970508 and find that the fraction of the energy in the magnetic field, epsilon_B < 6x10^-5, is much lower in the case of GRB 980703, which is a consequence of the high frequency of the cooling break.

We present a Keck II LRIS spectroscopic follow-up study of the possible optical counterparts to a flux-limited sample of galaxies selected from an 850-micron survey of massive lensing clusters using the SCUBA bolometer array on the JCMT. These sources represent a population of luminous dusty galaxies responsible for the bulk of the 850-micron background detected by COBE and thus for a substantial fraction of the total far-infrared emission in the Universe. We present reliable redshifts for 20 galaxies and redshift limits for a further 4 galaxies selected from the error-boxes of 14 submm sources. Two other submm detections in the sample have no obvious optical counterparts, and the final submm source was only identified from imaging data after the completion of our spectroscopic observations. The optical identifications for 4 of the submm sources have been confirmed through either their detection in CO at mm-wavelengths (two pairs of galaxies at z=2.55 and z=2.80) or from the characteristics of their spectral energy distributions (two of the central cD galaxies in the lensing clusters). Plausible arguments based on the optical spectral properties (starburst or AGN signatures) of the counterparts allow us to identify a further two likely counterparts at z=1.06 and 1.16. For the remaining 8 cases, it is not always clear which, if any, of the optical sources identified are the true counterparts. Possible counterparts for these have redshifts ranging from z=0.18 to z=2.11. Working with the current identifications, we suggest that the majority of the extragalactic background light in the submm is emitted by sources at z<3 and hence that the peak activity in highly-obscured sources (both AGN and starbursts) lies at relatively modest redshifts. (Abridged) (417kb)

We report on 450- and 850-um observations of the interacting galaxy pair, VV114E+W (IC1623), taken with the SCUBA camera on the James Clerk Maxwell Telescope, and near-infrared observations taken with UFTI on the UK Infrared Telescope. The system VV114 is in an early stage of a gas-rich merger. We detect submillimeter (sub-mm) emission extended over 30" (12 kpc) and find a good correlation between the spatial distribution of the sub-mm and CO emission. Both the CO and sub-mm emission peak near the reddest region of VV114E and extend toward VV114W. The bulk of the sub-mm emission resides in the central region showing the largest CO velocity gradients, which is thought to mark the kinematic centroid of the merger remnant. We derived a total dust mass of 1.2x10^8 Mo, assuming a power-law distribution of dust temperatures. The sub-mm observations suggest that the majority of the dust is relatively cool (Td~20-25K), and the total dust mass is about 4 times higher than that inferred from the IRAS data alone. The system will likely evolve into a compact starburst similar to Arp220.

We present new near-IR and optical spectroscopic observations which confirm the redshift of the z=1.44 extremely red object ERO J164502+4626.4 (object 10 of Hu & Ridgway 1994) and a HST image which reveals a reflected-S-shaped morphology at (rest-frame) near-UV wavelengths. The contrast between the rest-frame far-red (8200-9800A) and near-UV (2900-3900A) morphologies suggests that the central regions of the galaxy are heavily obscured by dust and that the galaxy is most likely an interacting or disturbed system. We also present new photometry of this object at 450, 850 and 1350 microns obtained using SCUBA on the JCMT. The rest-frame SED of this ERO is best understood in terms of a highly reddened stellar population with ongoing star formation, as originally suggested by Graham & Dey (1996). The new sub-mm data presented here indicate that the remarkable similarity to ultraluminous infrared galaxies (ULIRGs) such as Arp220 extends into the rest-frame far-IR which bears the signature of thermal emission from dust. ERO J164502+4626.4 is extremely luminous (7E12 Lsun) and dusty (M[dust] = 7E8 Msun). If its luminosity is powered by young hot stars, then ERO J164502+4626.4 is forming stars at the prodigious rate of 1000-2000 Msun/yr. We conclude that it is a distant analogue of the nearby ULIRG population, the more distant or less luminous counterparts of which may be missed by even the deepest existing optical surveys. The sub-mm emitters recently discovered by deep SCUBA surveys may be galaxies similar to ERO J164502+4626.4 (but perhaps more distant). This population of extremely dusty galaxies may also contribute significantly to the cosmic sub-mm background emission.

We present data from the SCUBA submillimetre camera for the central galaxies of seven concentrated clusters of galaxies at redshifts between 0.19 and 0.41. We detect submm emission from the central galaxies in the rich clusters A1835 and A2390, and present upper limits for the central galaxies in the remaining clusters. The two galaxies which we detect both exhibit unusually blue UV-optical colours and lie in clusters which contain massive cooling flows, >1000Mo/yr. Moreover, both galaxies contain strong radio sources. Focusing on these two systems, we present new and archival radio-optical observations to provide a detailed view of their spectral energy distributions. Our analysis indicates that the submm emission from the central galaxy of A1835 can be best understood as arising from dust, heated by vigorous star formation or an obscured active galactic nucleus. For the central galaxy of A2390, the submm flux is only barely consistent with an extrapolation of the centimetre-millimetre emission from the luminous radio source that lies its core; the submm excess is comparable to the total submm emission from A1835 and may also be due to emission from dust, although this statement requires confirmation. We present details of our multi-wavelength observations and discuss the implications of these data for the interpretation of star formation in cooling-flow galaxies.

We present spectroscopic observations of galaxies in the fields of 10 distant clusters for which we have previously presented deep imaging with WFPC2 on board the Hubble Space Telescope. The clusters span the redshift range z=0.37-0.56 and are the subject of a detailed ground- and space-based study to investigate the evolution of galaxies as a function of environment and epoch. The data presented here include positions, photometry, redshifts, spectral line strengths and classifications for 657 galaxies in the fields of the 10 clusters. The catalog comprises 424 cluster members across the 10 clusters and 233 field galaxies, with detailed morphological information from our WFPC2 images for 204 of the cluster galaxies and 71 in the field. We illustrate some basic properties of the catalog, including correlations between the morphological and spectral properties of our large sample of cluster galaxies. A direct comparison of the spectral properties of the high redshift cluster and field populations suggest that the phenomenon of strong Balmer lines in otherwise passive galaxies (commonly called E+A, but renamed here as the k+a class) shows an order-of-magnitude increase in the rich cluster environment, compared to a more modest increase in the field population. This suggests that the process or processes involved in producing k+a galaxies are either substantially more effective in the cluster environment or that this environment prolongs the visibility of this phase. A more detailed analysis and modeling of these data will be presented in Poggianti et al. (1998).

We present a detailed analysis of the spectroscopic catalog of galaxies in 10 distant clusters from Dressler et al. (1999, D99). We investigate the nature of the different spectral classes defined by D99 including star forming, post-starburst and passive galaxy populations, and reproduce their basic properties using our spectral synthesis model. We attempt to identify the evolutionary pathways between the various spectral classes in order to search for the progenitors of the numerous post-starburst galaxies. The comparison of the spectra of the distant galaxy populations with samples drawn from the local Universe leads us to identify a significant population of dust-enshrouded starburst galaxies, showing both strong Balmer absorption and relatively modest [OII] emission, that we believe are the most likely progenitors of the post-starburst population. We present the differences between the field and cluster galaxies at z=0.4-0.5. We then compare the spectral and the morphological properties of the distant cluster galaxies, exploring the connection between the quenching of star formation inferred from the spectra and the strong evolution of the S0 population discussed by Dressler et al. (1997). We conclude that either two different timescales and/or two different physical processes are responsible for the spectral and the morphological transformation.

We present the first results on the identification and study of very distant field galaxies in the core of cluster-lenses, using a selection criterium based on both lens modelling and photometric redshifts. We concentrate on two multiple-imaged sources at z=4.05 in the cluster A2390. The 2 objects presented in this paper, namely H3 and H5, were identified through lens modelling as multiple images of high-redshift sources at z>3.5. We confirm the excellent agreement between this identification and both their photometric redshifts and morphologies. Our CFHT/WHT program for a systematic redshift survey of arcs in clusters has allowed to obtain a set of spectra on 3 different images at z~4: the brightest image of H3, which redshift was already confirmed by Frye & Broadhurst (1998), and the two brightest images of H5. The later is then confirmed spectroscopically as a multiple image, giving a strong support to the lens model. The main feature in each of these spectra is a strong emission line, identified as Ly-alpha, leading to z=4.05 for both H3 and H5. The spectrophotometric properties of these galaxies are studied, in particular the degeneracy in the parameter-space defined by the SFR type, age, metallicity and reddening. H3 and H5 are intrinsically bright and clumpy sources located ~100 kpc part on the source plane, with mean metallicities compatible with a fraction of solar or even solar values. All these results seem to favour a hierarchical merging scenario, where we are actually seeing a relatively advanced step for these 2 z~4 objects, with stars forming locally and efficiently from a preenriched gas.

We report the detection of CO(3-2) emission from the submillimeter-selected luminous galaxy SMM J14011+0252. The optical counterpart of the submillimeter source has been identified as a merger system with spectral characteristics consistent with a starburst at z=2.565. The CO emission confirms the optical identification of the submillimeter source and implies a molecular gas mass of 5*10^10 (h_75)^-2 Mo, after correcting for a lensing amplification factor of 2.75. The large molecular gas mass and the radio emission are consistent with the starburst interpretation of the source. These results are similar to those found for SMM J02399-0136, which was the first submillimeter selected CO source found at high redshift. The CO detections of these two high-redshift submillimeter galaxies suggest the presence of massive reservoirs of molecular gas which is consistent with the inferred high rates of star-formation (1000 Mo/yr). These two systems appear to be associated with merger events which may evolve into present day luminous elliptical galaxies.

We present the counts of luminous submillimeter (sub-mm) galaxies from an analysis of our completed survey of the distant Universe seen through lensing clusters. This survey uses massive clusters lenses with well-constrained mass models to obtain a magnified view of the background sky. This both increases the sensitivity of our sub-mm maps and reduces the effects of source confusion. Accurate lens models are used to correct the observed sub-mm source counts for the lens amplification. We show that the uncertainties associated with this correction do not dominate the final errors. We present sub-mm counts derived from two independent methods: a direct inversion of the observed sources, which are corrected individually for lens amplification; and a Monte-Carlo simulation of our observations using a parametric model for the background counts, which is folded through the lens models and incompleteness estimates to determine best-fitting values of the count parameters. Both methods agree well and confirm the robustness of our analysis. Detections that are identified with galaxies in the lensing clusters in deep optical images (Smail et al. 1998) are removed prior to our analysis, and the results are insensitive both to the details of the correction and to the redshift distribution of the detections. We present the 850-um counts at flux densities between 0.5 and 8mJy. The count of galaxies brighter than 4mJy is (1600+/-700) per sq. deg, in agreement with the value of (2500+/-1400) per sq. deg reported by Smail, Ivison & Blain (1997). The most accurate 850-um count is determined at 1mJy: (8000+/-2800) per sq. deg. All quoted errors include both Poisson and systematic terms. These are the deepest sub-mm counts published, and are not subject to source confusion because the detected galaxies are separated and magnified by the lens. Down to the 0.5-mJy limit of our counts, the resolved 850-um background radiation intensity is (5+/-2) x 10^-10 W/m^2/sr, comparable to the current COBE estimate of the background. This indicates that the bulk of the 850-um background radiation originated in distant ultraluminous galaxies.

A population of distant dusty galaxies emitting in the submillimetre waveband has recently been detected using the Submillimetre Common-User Bolometer Array (SCUBA) camera on the James Clerk Maxwell Telescope (JCMT). This population can be used to trace the amount of high-redshift star formation activity that is obscured from view in the optical waveband by dust, and so is missing from existing inventories of star formation in the distant Universe. By includling this population we can construct a complete and consistent picture of the history of star formation. The evolution of obscured star formation at redshifts less than unity is constrained by mid- and far-infrared counts of dusty galaxies. Activity increases with redshift z as (1+z)^gamma with gamma~4, consistent with the form of evolution found in the optical by the Canada-France Redshift Survey (CFRS) to z~1. The form of evolution at higher redshifts is constrained by both our faint SCUBA counts and the intensity of background radiation in the millimetre/submillimetre waveband. We find that the total amount of energy emitted by dusty galaxies is about five times greater than that inferred from restframe UV observations, and that a larger fraction of this energy is emitted at high redshifts. The simplest explanation for these results is that a large population of luminous strongly-obscured sources at redshifts of z<5 is missing from optical surveys. We discuss the possible contribution of obscured active galactic nuclei to the submillimetre-wave background and counts. More accurate constraints on the history of star formation will be provided by determinations of the counts in several submillimetre wavebands and crucially, by a reliable redshift distribution of the detected galaxies.

We present the identifications of two z~3.8 quasars from a deep UBI imaging survey with the Palomar 5.1-m. The survey covers an area of 0.25 sq. degrees around a sample of 10 z=0.2-0.3 luminous X-ray clusters. The QSOs were identified on the basis of their stellar morphologies, relatively blue optical and very red UV-optical colours. The two objects are Q1322+5034 with total magnitudes of B=20.8, I=18.3 and (U-B)>4.7; and Q1722+3211 which has total magnitudes of B=21.8, I=19.7 and (U-B)>3.2. Subsequent spectroscopic observations with the 4.2-m William Herschel Telescope have confirmed the identity of these two sources as QSO at z=3.82 and z=3.73 respectively. Our spectroscopic observations identify a damped Lyman-alpha absorber in the spectrum of Q1322+5034 at z=3.439, as well as a second absorption system at z=2.700 which may either be a single very high column-density damped Lyman-alpha system, or more likely a blend of a number of high column-density absorbers spread over a distance of ~10 Mpc.

We present deep optical imaging from Hubble Space Telescope (HST) and the ground for galaxies selected from the first sub-millimeter (sub-mm) survey of the distant Universe. Our survey comprises 850-um continuum maps of seven massive lensing clusters and covers a total area of 0.01 sq. deg to 1-sigma noise levels of <= 2 mJy/beam. We detect 17 sources brighter than the 50% completeness limits (10 brighter than the 80% limit). Of the sources which lie within our optical fields, counterparts are identified for 14 of the 16 sources in the f(50%) sample (9/10 in the f(80%) sample). The morphologies of those galaxies for which we have {\it HST} imaging fall into three broad categories: faint disturbed galaxies and interactions; faint galaxies too compact to classify reliably; and dusty, bright galaxies at intermediate redshifts. The disturbed and interacting galaxies constitute the largest class, which suggests that interactions remain an important mechanism for triggering star formation in ultraluminous galaxies in the distant Universe. The faint, compact galaxies may represent a later evolutionary stage in these mergers, or more centrally-concentrated starbursts. It is likely that some of these will host active galactic nuclei. Analysis of the colors of our sample allow us to place a crude limit on the redshift distribution: >75% have z<5.5 whilst >50% lie at z<4.5, suggesting that the luminous sub-mm population is broadly coeval with the more modestly star-forming galaxies selected by UV/optical surveys of the distant Universe.

We report the detection of CO(3->2) emission from the submillimeter-selected hyperluminous galaxy SMM02399-0136. This galaxy is the brightest source detected in the recent survey of the submillimeter sky. The optical counterpart of the submillimeter source has been identified as a narrow-line AGN/starburst galaxy at z=2.8. The CO emission is unresolved, < 5 arcsec, and is coincident in redshift and position with the optical counterpart. The molecular gas mass derived from the CO observations is 8 x 10^10 Mo (h=0.75), after correcting for a lensing amplification factor of 2.5. The large CO luminosity indicates that a significant fraction of the infrared luminosity of SMM02399-0136 arises from star-formation. The high inferred star-formation rate of 1000 Mo/tr and the large gaseous reservoir may suggest that we are seeing the formation phase of a massive galaxy. Future CO observations of other galaxies detected in deep SCUBA surveys will test the generality of these conclusions for the bulk of the faint submillimeter population.

We present a detailed study of SMM02399-0136, a hyperluminous, active galaxy selected from a sub-mm survey of the distant Universe. This galaxy is the brightest source in the fields of six rich, lensing clusters, with a total area of ~0.01 per sq. degree, that we have mapped with a sensitivity of ~2 mJy/beam at 850um. We identify a compact optical counterpart with an apparent magnitude of B~23 and a low-surface-brightness companion ~3 arcsec away. Our spectroscopy shows that both components have the same redshift; z = 2.803+/-0.003. The emission line widths, FWHM ~ 1000-1500 km/s, and line ratios, along with the compact morphology and high luminosity (M_B ~ -24.0) of the galaxy indicate that SMM02399-0136 contains a rare dust-embedded, narrow-line or type-2 active galactic nucleus (AGN). The source is gravitationally lensed by the foreground cluster, amplifying its apparent luminosity by a factor of 2.5, and our detailed lens model allows us to accurately correct for this. Taking the amplification into account we estimate that SMM02399-0136 is intrinsically a factor of five times more luminous than IRAS F10214+4724. Its far-infrared and H-alpha luminosities and low-surface-brightness radio emission are indicative of an interaction-induced starburst and the star-formation rate (SFR) could be several thousand solar masses per year. This, however, assumes that the starburst is the dominant source of energy, but we cannot yet determine reliably the relative contributions of the starburst and the buried AGN. A dust mass of 5-7 * 10^8 Mo is indicated by our data for a dust temperature of 40-50k, independent of the dominant energy source. We estimate the possible space density of such luminous sub-mm sources, and find that while a large population of these obscured sources could be detected in future wide-field sub-mm surveys, they are unlikely to dominate the faint counts in this waveband. Galaxies such as SMM02399-0136 and F10214+4724 cannot be easily detected in conventional AGN/QSO surveys, and so estimates of the prevalence of AGN in the early Universe may require significant revision.

We analyze surface photometry measurements of elliptical galaxies in the cores of 13 rich clusters with redshifts between z=0.17 and z=0.56. These galaxies have been morphologically classified from deep Hubble Space Telescope observations. We also present new near-infrared images over the same fields for a substantial subset. We find no evidence for strong evolution in the K-band galaxy luminosity functions between z=0.31 and z=0.56. Using curve-of-growth de Vaucoleurs fits to the Hubble Space Telescope elliptical galaxy images, we measure effective metric radii, R_e, and mean effective surface brightnesses, < u_e >. The variation with redshift of the < u_e > values at a standard condition, such as R_e=1 kpc, in both the B and K bands is found to be consistent with passive evolution expectations in which the bulk of the stellar population has formed by redshifts z_f>2.

We present a new wide field image of the distant cluster AC114 (z=0.31) obtained with Wide Field and Planetary Camera II onboard the Hubble Space Telescope. This image considerably extends our knowledge of the lensing properties of the cluster beyond those derived by Smail et al. (1995a) from a single WF/PC-1 pointing. In conjunction with published ground-based spectroscopy, we utilise several newly-discovered multiple images to construct an improved mass model for the central regions of the cluster. Using this model, we apply the methodology introduced by Natarajan & Kneib (1997) to interpret local perturbations to the cluster shear field on small scales resulting from mass associated with individual cluster galaxies. We use the lensing signal to place new constraints on the average mass-to-light ratio and spatial extents of the dark matter halos associated with morphologically-classified early-type cluster members. We find that the total mass of a fiducial L* cluster spheroidal galaxy is largely contained within ~15 kpc radius halo (~8-10 R_e) with a mass-to-light ratio M/L_V ~ 15^+10_-4 (90% c.l., h=0.5) in solar units within this radius. Comparisons with similar estimates for field galaxies indicates that the cluster galaxies in AC114 may possess less extensive and less massive halos. We discuss the consequences of our result in the context of models for the dynamical evolution of cluster galaxies and the observational prospects for extending this analysis.

The first observations to detect a population of distant galaxies directly in the submillimetre waveband have recently been made using the new Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). The results indicate that a large number of distant galaxies are radiating strongly in this waveband. Here we discuss their significance for source confusion in future millimetre/submillimetre-wave observations of both distant galaxies and cosmic microwave background radiation (CMBR) anisotropies. Earlier estimates of such confusion involved significant extrapolation of the results of observations of galaxies at small redshifts; however, our new estimates do not, as they are derived from direct observations of distant galaxies in the submillimetre waveband. The results have important consequences for the design and operation of existing and proposed millimetre/submillimetre-wave telescopes: the Planck Surveyor mission will be confusion-limited at frequencies greater than 350 GHz, even in the absence of galactic dust emission; 1-sigma confusion noise of 0.6 mJy/beam is expected for the JCMT/SCUBA at a wavelength of 850um; and the sub-arcsecond resolution of large millimetre/submillimetre-wave interferometer arrays will be required in order to execute very deep galaxy surveys.

The angular clustering of faint field galaxies is investigated using deep imaging (I~25) obtained with the 10-m Keck-I telescope. The autocorrelation function is consistent with W = theta^-0.8 and we find no compelling evidence for a flattening of the power law index at the faintest magnitude limits. Results from a number of independent observational studies are combined in order to investigate the variation of the correlation amplitude with median I-magnitude. At Imed ~ 23 the results obtained by different studies are all in rough agreement and indicate that for Imed > 22 the correlation amplitude declines far less steeply than would be expected from an extrapolation of the trend in the brighter samples. In particular, at Imed ~ 24 our data indicate W to be a factor ~7 higher than the extrapolation. A near-independence of magnitude is a general feature of the correlation amplitude in models in which the redshift distribution of the faint field population contains a substantial fraction of galaxies with z > 1. In order to reproduce the apparent abrupt flattening of the amplitude of W observed at faint limits, approximately 50% of the galaxies in a sample with a depth of I ~ 25 must be at z > 1.

We present new results of a program to study the detailed morphologies of galaxies in intermediate redshift clusters and hence understand the physical origin of the enhanced star formation in these environments at earlier epochs. Deep, high resolution imagery has been obtained of 3 rich clusters, AC103, AC118 & AC114 at z=0.31, using the WFPC2 on HST. For AC103 & AC118, single pointings covering a central 0.5×0.5 Mpc have been obtained; for AC114, 4 pointings covering a 1.2×0.7 Mpc area have been obtained, allowing the more outer regions of a distant cluster to be studied. Hubble types plus evidence for dynamical interactions and/or structural abnormalities have been determined visually for all galaxies down to R=22.25 in AC103 & AC118 and R=23.0 in AC114. We find the numbers of spirals (Sa-Sdm) in our clusters to be up to 4× higher than that seen in present-day clusters; only in the virialised core of our most massive regular cluster, AC114, do we see morphological fractions approaching those of the present epoch. Dynamical interactions are also widespread throughout the clusters with ~20% of members showing morphological evidence of this phenomenon. The highest incidence is amongst those blue members either undergoing a starburst or seen < 0.5 Gyr thereafter with the majority being involved in major mergers. These galaxies, however, are of modest luminosity (L ~ L* + 1 mag), destined to become dwarfs once they fade. Cluster members with ongoing star formation typical of nearby spirals are largely late Sb-Sdm Hubble types. Galaxies whose star formation ended 1-2 Gyr prior to the epoch of observation are all normal early-type (S0-Sb) disk systems. The red H-delta strong galaxies are a mixture of E & S0 galaxies, consistent with them being old dormant systems which have undergone a secondary star formation episode.

We present deep spectroscopic measurements of 18 distant field galaxies identified as gravitationally-lensed arcs in a Hubble Space Telescope image of the cluster Abell 2218. Redshifts of these objects were predicted by Kneib et al. (1996) using a lensing analysis constrained by the properties of two bright arcs of known redshift and other multiply-imaged sources. The new spectroscopic identifications were obtained using long exposures with the LDSS-2 spectrograph on the William Herschel Telescope and demonstrate the capability of that instrument to new limits, R~24; the lensing magnification implies true source magnitudes as faint as R~25. Statistically, our measured redshifts are in excellent agreement with those predicted from Kneib et al.'s lensing analysis which gives considerable support to the redshift distribution derived by the lensing inversion method for the more numerous and fainter arclets extending to R~25.5. We explore the remaining uncertainties arising from both the mass distribution in the central regions of Abell 2218 and the inversion method itself, and conclude that the mean redshift of the faint field population at R~25.5 (B~26-27) is low, < z >=0.8-1. We discuss this result in the context of redshift distributions estimated from multi-colour photometry. Although such comparisons are not straightforward, we suggest that photometric techniques may achieve a reasonable level of agreement particularly when they include near-infrared photometry with discriminatory capabilities in the 1 < z < 2 range.

We present a deep UBI CCD survey using the Palomar 5-m telescope of a sample of high X-ray luminosity, distant clusters selected from the ROSAT All-Sky Survey. The 10 clusters lie at z=0.22-0.28, an era where evolutionary effects have been reported in the properties of cluster galaxy populations. Our clusters thus provide a well-defined sample of the most massive systems at these redshifts to quantify the extent and variability of these evolutionary effects. Moreover, by concentrating on a narrow redshift range, we can take advantage of the homogeneity of our sample to combine the catalogues from all the clusters to analyse the bulk properties of their populations. The core regions of these clusters contain only a small proportion of star-forming galaxies, and they therefore do not exhibit a `Butcher-Oemler' effect. Focusing on the redder cluster galaxies we find that their integrated luminosity is well correlated with the cluster X-ray temperatures, and hence with cluster mass. Furthermore, the typical UV-optical colours of the elliptical sequences in the clusters exhibit a small cluster-to-cluster scatter, <= 2%, indicating that these galaxies are highly homogeneous between clusters. However, at fainter magnitudes we observe an increase in the range of mid-UV colours of galaxies possessing strong 4000A breaks. In the light of the apparent decline in the S0 populations of z >= 0.4 clusters (Dressler et al. 1997), and in view of the luminosities and colours of this population, we propose that they may be the progenitors of the S0 population of local rich clusters, caught in the final stage before they become completely quiescent.

Using traditional morphological classifications of galaxies in 10 intermediate-redshift (z~0.5) clusters observed with WFPC2 on the Hubble Space Telescope, we derive relations between morphology and local galaxy density similar to that found by Dressler for low-redshift clusters. Taken collectively, the `morphology-density' relationship, M-D, for these more distant, presumably younger clusters is qualitatively similar to that found for the local sample, but a detailed comparison shows two substantial differences: (1) For the clusters in our sample, the M-D relation is strong in centrally concentrated ``regular'' clusters, those with a strong correlation of radius and surface density, but nearly absent for clusters that are less concentrated and irregular, in contrast to the situation for low redshift clusters where a strong relation has been found for both. (2) In every cluster the fraction of elliptical galaxies is as large or larger than in low-redshift clusters, but the S0 fraction is 2-3 times smaller, with a proportional increase of the spiral fraction. Straightforward, though probably not unique, interpretations of these observations are (1) morphological segregation proceeds hierarchically, affecting richer, denser groups of galaxies earlier, and (2) the formation of elliptical galaxies predates the formation of rich clusters, and occurs instead in the loose-group phase or even earlier, but S0's are generated in large numbers only after cluster virialization.

We present the first results of a sub-millimeter survey of distant clusters using the new Sub-mm Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. We have mapped fields in two massive, concentrated clusters, A370 at z=0.37 and Cl2244-02 at z=0.33, at wavelengths of 450 and 850-um. The resulting continuum maps cover a total area of about 10 sq. arcmin to 1 sigma noise levels less than 14 and 2 mJy per beam at the two wavelengths, 2-3 orders of magnitude deeper than was previously possible. We have concentrated on lensing clusters to exploit the amplification of all background sources by the cluster, improving the sensitivity by a factor of 1.3-2 as compared with a blank-field survey. A cumulative source surface density of (2.4+/-1.0) x 10^3 per sq. degree is found to a 50% completeness limit of ~4 mJy at 850-um. The sub-mm spectral properties of these sources indicate that the majority lie at high redshift, z>1. Without correcting for lens amplification, our observations limit the blank-field counts at this depth. The surface density is 3 orders of magnitude greater than the expectation of a non-evolving number density of strongly star-forming galaxies in the high-redshift Universe and suggest that optical surveys may have substantial underestimated the star formation density in the distant Universe. Deeper sub-mm surveys with SCUBA should detect large numbers of star-forming galaxies at high redshift, and so provide strong constraints on the formation of normal galaxies.

In this paper we present weak lensing observations of the fields around 8 z~1 luminous radio sources. These data are searched for the lensing signatures of clusters that are either physically associated with the radio objects, or are foreground systems projected along the line of sight. The radio sources were all imaged with WFPC2/HST providing high quality shape information on large numbers of faint galaxies around them. Statistical analysis of the coherent shear field visible in the shapes of the faint galaxies indicates that we have detected a weak lensing signal close to one of the targets, 3C336 at z=0.927, with a high level of confidence. A second, independent WFPC2 observation of this target reinforces this detection. Our results support the earlier suggestion of weak lensing in this field by Fort et al (1996) using ground-based data. We also combined the shear distributions in the remaining 7 field to improve our sensitivity to weak shear signals from any structure typically associated with these sources. We find no detectable signal and estimate an upper limit on the maximum shear allowed by our observations. Using an N(z) estimated from lensing analyses we convert our observed lensing signal and limits into estimates of the masses of the various structures. We suggest that further lensing observations of distant radio sources and their host environments may allow the cluster L_X-mass relationship to be mapped at high-z. This is crucial for interpreting the results of the next generation of deep X-ray surveys, and thus constraining the redshift evolution of the cluster mass function out to z=1.

Ground-based counts and colors of faint galaxies in the U and R bands in one field at high Galactic latitude are presented. Integrated over flux, a total of 1.2x10^5 sources per square degree are found to U=25.5 mag and 6.3x10^5 sources per square degree to R=27 mag, with d log N/dm ~ 0.5 in the U band and d log N/dm ~ 0.3 in the R band. Consistent with these number-magnitude curves, sources become bluer with increasing magnitude to median U-R=0.6 mag at 24< U < 25 mag and U-R=1.2 mag at 25 < R < 26 mag. Because the Lyman break redshifts into the U band at z~3, at least 1.2x10^5 sources per square degree must be at redshifts z < 3. Measurable U-band fluxes of 73 percent of the 6.3x10^5 sources per square degree suggest that the majority of these also lie at z < 3. These results require an enormous space density of objects in any cosmological model.

The small scatter observed for the U-V colors of spheroidal galaxies in nearby clusters of galaxies provides a powerful constraint on the history of star formation in dense environments. However, with local data alone, it is not possible to separate models where galaxies assembled synchronously over redshifts 0 < z < 1 from ones where galaxies formed stochastically at much earlier times. Here we attempt to resolve this ambiguity via high precision rest-frame UV-optical photometry of a large sample of morphologically-selected spheroidal galaxies in three z~0.54 clusters which have been observed with HST. We demonstrate the robustness of using HST to conduct the morphological separation of spheroidal and disk galaxies at this redshift and use our new data to repeat the analysis conducted locally at a significant look-back time. We find a small scatter (< 0.1 mag rms) for galaxies classed as Es and E/S0s, both internally within each of the three clusters and externally from cluster to cluster. We do not find any trend for the scatter to increase with decreasing luminosity down to L = L*+3, other than can be accounted for by observational error. Neither is there evidence for a distinction between the scatter observed for galaxies classified as ellipticals and S0. Our result provides a new constraint on the star formation history of cluster spheroidals prior to z=0.5 confirming and considerably strengthening the earlier conclusions. Most of the star formation in the elliptical galaxies in dense clusters was completed before z~3 in conventional cosmologies. Although we cannot rule out the continued production of some ellipticals, our results do indicate an era of initial star formation consistent with the population of star-forming galaxies recently detected beyond z~3.

We present catalogs of objects detected in deep images of 11 fields in 10 distant clusters obtained using WFPC2 on board the Hubble Space Telescope. The clusters span the redshift range z=0.37-0.56 and are the subject of a detailed ground- and space-based study to investigate the evolution of galaxies as a function of environment and epoch. The data presented here include positions, photometry and basic morphological information on ~9000 objects in the fields of the 10 clusters. For a brighter subset of 1857 objects in these areas we provide more detailed morphological information.

We present weak lensing results for 12 distant clusters determined from images obtained with the refurbished HST. We detect the signature of gravitational lensing in 11 of the 12 clusters; the clusters span nearly an order of magnitude in lensing strength. The sample thus provides an excellent database for correlating direct mass estimates from lensing with indirect ones which rely on baryonic tracers. We examine the correlation between the cluster X-ray luminosities and the mean gravitational shear strengths and develop a model which predicts the relationship expected from the properties of local clusters. After allowing for various observational effects, we find that the predicted correlation is a reasonable match to the available data, indicating that there has been little evolution in the X-ray luminosity-central mass relationship between z=0.4 and now. We discuss the implications of this result in the context of the evolution of the X-ray luminosity function found by earlier workers. The comparison between shear amplitudes and velocity dispersions, estimated from a modest sample of members, reveals a discrepancy in the sense that these velocity dispersions are typically over-estimated by factors of ~50%. This supports earlier suggestions that high dispersions measured for distant clusters may be seriously affected by both unidentified substructure and outliers. Combining our lensing masses with morphologically-based luminosity estimates, we determine mass/light ratios in solar units of M/L_V=180+/-160 h for the entire population and 620+/-250 h for the spheroidal galaxies where the evolutionary effects can be best treated. We argue that this provides an upper bound to the local cluster M/L corresponding to Omega~0.4.

We present deep two-colour photometry of two rich clusters at z=0.18, A665 and A1689. We use these data to construct number counts as a function of magnitude in the two fields. By combining these counts with similar observations from a large area field survey we subtract the field contamination statistically to produce luminosity functions for the two clusters. Great care has been taken to achieve agreement between the photometry of these two samples. The cluster data are complete to a limiting magnitude of I=22.5 or an absolute magnitude in the cluster of I=-18.0 (M*+5). The luminosity functions of both clusters are well described by a Gaussian function for the bright galaxies, combined with a Schechter function at the faint end, similar to that required to fit the luminosity function in local clusters. The slope at the faint end of the Schechter function in both clusters is extremely steep in V, alpha~-2. A shallower slope is seen to the limit of the I data, indicating that the cluster population is rapidly blueing as we reach fainter. The excellent agreement between the form of the luminosity function in our two distant clusters, as well as agreement with the luminosity function given by Driver et al. (1994) for a single z=0.21 cluster, indicates that this faint blue population is a general constituent of distant clusters. We compare our results with those from studies of local clusters. Depending upon the degree of fading (or disruption) of these faint blue galaxies,we tentatively identify their remnants with the low surface brightness dwarf galaxies which are the dominant population in local clusters. We discuss the possible role of this population as the source of most of the X-ray gas in rich clusters.

We have combined deep starcount data with Galaxy model predictions to investigate how effectively such measurements probe the faint end of the halo luminosity function. We have tested a number of star/galaxy classification techniques using images taken in 0.5 arcsecond seeing with LRIS on the Keck telescope, and we find that different combinations of these techniques can produce variations of 10% in the inferred starcounts at R=22.5 and 30% at R=24.5 magnitudes. The decreasing average angular size of galaxies with fainter magnitude effectively limits ground-based work to R < 25.5 magnitudes. The higher angular resolution provided by HST allows one to probe at least 2 magnitudes fainter, but the small field-size is a significant limitation. In either case, our models show that the contribution from halo subdwarfs is effectively limited to colours of (R-I) < 1.0, with the redder stars being members of the Galactic disk. The apparent increase in number density for M_V > 10 in the derived luminosity function is a result of contributions from disk stars at fainter absolute magnitudes and does not provide evidence for an upturn in the halo subdwarf mass function. Indeed, starcount data alone are not an effective method of probing the shape of the halo luminosity function close to the hydrogen-burning limit. Finally, we examine how the Hubble Deep Field observations can be used to constrain the contribution of various stellar components to the dark-matter halo.

We present a striking new Hubble Space Telescope (HST) observation of the rich cluster Abell 2218 taken with the Wide-Field and Planetary Camera (WFPC2). HST's restored image quality reveals a sizeable number of gravitationally-lensed features in this cluster, significantly more than had been identified using ground-based telescopes. The brightest arcs are resolved by HST and show internal features enabling us to identify multiply-imaged examples, confirming and improving the mass models of the cluster determined from ground-based observations. Although weak lensing has been detected statistically in this and other clusters from ground-based data, the superlative resolution of HST enables us to individually identify weakly distorted images more reliably than hitherto, with important consequences for their redshift determination. Using an improved mass model for the cluster calibrated with available spectroscopy for the brightest arcs, we demonstrate how inversion of the lensing model can be used to yield the redshift distribution of ~80 faint arclets to R~25. We present a new formalism for estimating the uncertainties in this inversion method and review prospects for interpreting our results and verifying the predicted redshifts.

We present HST imaging of eight spectroscopically-confirmed giant arcs, pairs and arclets. These objects have all been extensively studied from the ground and we demonstrate the unique advantages of HST imaging in the study of such features by a critical comparison of our data with the previous observations. In particular we present new estimates of the core radii of two clusters (Cl0024+16, A370) determined from lensed features which are identifiable in our HST images. Although our HST observations include both pre- and post-refurbishment images, the depth of the exposures guarantees that the majority of the arcs are detected with diffraction-limited resolution. A number of the objects in our sample are multiply-imaged and we illustrate the ease of identification of such features when working at high resolution. We discuss the morphological and scale information on these distant field galaxies in the light of HST studies of lower redshift samples. We conclude that the dominant population of star-forming galaxies at z=1 is a factor of 1.5-2 times smaller than the similar group in the local field. This implies either a considerable evolution in the sizes of star-forming galaxies within the last ~10 Gyrs or a shift in the relative space densities of massive and dwarf star-forming systems over the same timescale.

We discuss the optical spectrum of a multiply-imaged arc resolved by HST in the z=0.175 cluster A2218. The spectrum, obtained with LDSS-2 on the 4.2m William Herschel telescope, reveals the source to be a galaxy at a redshift z=2.515 in excellent agreement with the value predicted by Kneib et al. (1996) on the basis of their inversion of a highly-constrained mass model for the lensing cluster. The source is extremely blue in its optical-infrared colours, consistent with active star formation, and the spectrum reveals absorption lines characteristic of a young stellar population. Of particular significance is the absence of Lyman-alpha emission but the presence of a broad Lyman-alpha absorption. The spectrum is similar to that of other, much fainter, galaxies found at high redshift by various techniques and illustrates the important role that lensing can play in detailed studies of the properties of distant galaxies.

We report a significant detection of weak, tangential distortion of the images of cosmologically distant, faint galaxies due to gravitational lensing by foreground galaxies. A mean image polarisation of P=0.011+/-0.006 is measured for 3202 pairs of source galaxies with magnitudes 23 < r < 24 and lens galaxies with magnitudes 20 < r < 23. The signal remains strong for lens-source separations < 90'', consistent with quasi-isothermal galaxy halos extending to large radii ( > 100h^-1 kpc). Our observations thus provide the first evidence from weak gravitational lensing of large scale dark halos associated with individual galaxies. The observed polarisation is also consistent with the signal expected on the basis of simulations incorporating measured properties of local galaxies and modest extrapolations of the observed redshift distribution of faint galaxies. From the simulations we derive a best-fit halo circular velocity of V~220 km/s and characteristic radial extent of s > 100 h^-1 kpc. Our best-fit halo parameters imply typical masses for the lens galaxies within a radius of 100h^-1 kpc on the order of (1.0^+1.1_-0.7)x 10^12 h^-1 Mo, in good agreement with recent dynamical estimates of the masses of local spiral galaxies. This is particularly encouraging as the lensing and dynamical mass estimators rely on different sets of assumptions. Contamination of the gravitational lensing signal by a population of tidally distorted satellite galaxies can be ruled out with reasonable confidence. The prospects for corroborating and improving this measurement seem good, especially using deep HST archival data.

We analyse the detailed distribution of star-forming and post-starburst members in three distant (z=0.31) galaxy clusters in terms of evolutionary sequences that incorporate secondary bursts of star formation on pre-existing stellar populations. Using the number density of spectroscopically-confirmed members on the EW(H-delta) versus B-R plane from existing data, and for a larger K'-limited sample on the U-I versus I-K' plane from newly-acquired infrared images, we demonstrate that the proportion of cluster members undergoing secondary bursts of star formation during the last ~2 Gyr prior to the epoch of observation is probably as high as 30 per cent of the member galaxies. A key observation leading to this conclusion is the high proportion of H-delta strong galaxies in all three clusters. The evolutionary modelling, whilst necessarily approximate, returns the correct proportions of galaxies in various stages of the star formation cycle both in terms of spectral and colour properties. HST images for the three clusters indicate a high proportion of the active members show signs of interaction, whereas the H-delta strong galaxies appear mainly to be regular spheroidals. We examine results from recent merger simulations in the context of the populations in these clusters and confirm that the merging of individual galaxies, triggered perhaps by the hierarchical assembly of rich clusters at this epoch, is consistent with the star formation cycle identified in our data. The implications of such a high fraction of active objects in cluster cores is briefly discussed.

We present the detection of weak gravitational lensing in the field of the radio galaxy 3C324 (z=1.206) using deep HST imaging. From an analysis of the shapes of faint R=24.5-27.5 galaxies in the field we measure a weak, coherent distortion centered close to the radio source. This shear field most likely arises from gravitational lensing of distant field galaxies by a foreground mass concentration. In the light of previous observations of this region, which indicate the presence of a rich cluster around the radio source, we suggest that the most likely candidate for the lens is the cluster associated with the radio galaxy at z=1.2. If so, this is the most distant cluster to have been detected by weak shear observations. Such a statement has two important consequences. Firstly, it shows that massive, collapsed structures exist in the high redshift Universe, and secondly that a significant fraction of the R=24.5-27.5 field galaxy population lies beyond z=1.2.

We report the discovery with the Keck telescope of two new multiply-imaged arcs in the luminous X-ray cluster A2219 (z=0.225). The brighter arc in the field is red and we use spectroscopic and photometric information to identify it as a z~1 moderately star-forming system. The brightness of this arc implies that it is formed from two merging images of the background source, and we identify possible candidates for the third image of this source. The second giant arc in this cluster is blue, and while fainter than the red arc it has a similarly large angular extent (32 arcsec). This arc comprises three images of a single nucleated source - the relative parities of the three images are discernible in our best resolution images. The presence of several bright multiply imaged arcs in a single cluster allows detailed modelling of the cluster mass distribution, especially when redshift information is available. We present a lensing model of the cluster which explains the properties of the various arcs, and we contrast this model with the optical and X-ray information available on the cluster. We uncover significant differences between the distributions of mass and X-ray gas in the cluster. We suggest that such discrepancies may indicate an on-going merger event in the cluster core, possibly associated with a group around the second brightest cluster member. The preponderance of similar merger signatures in a large fraction of the moderate redshift clusters would indicate their dynamical immaturity.

We present faint galaxy counts from deep VRI images obtained with the Keck Telescope. These images reach R~27 in median seeing FWHM ~0.5-0.6 arcsec and we detect a integrated galaxy number density of 7x10^5 sq. degree, equivalent to 3x10^10 galaxies in the observable Universe. In addition we present median galaxy colors as a function of magnitude; bluing trends are visible in all colors to R~24.5. Fainter than R~24.5, however, the typical (V-R) color becomes redder again, (V-I) remains constant, and (R-I) becomes yet bluer. These trends are consistent with the VRI count slopes, implying a decrease in the V slope at the faintest levels, which our data supports. Taking advantage of our good seeing we also present median half-light radii for faint galaxies, these show a steady decline at fainter magnitudes, leading to an intrinsic half-light radius of ~0.2 arcsec for a typical R~25.5-26 galaxy. Irrespective of the redshift distribution the extremely high galaxy surface densities and their small intrinsic sizes are consistent with a scenario in which the majority of the very faint field population are dwarf galaxies or sub-galactic units.

We present results for the two-point angular correlation function of galaxies to a limiting magnitude of r=26. The final sample is 97% complete to r=26.0, yielding 5730 galaxies over a 90.1 sq. arcmin field. The correlation function for our faint galaxies can be parameterised by a power law, Aw theta^-0.8, in agreement with the clustering statistics of shallower catalogues. The derived amplitude, Aw, is small, but non-zero. We combine this measurement with the latest statistical constraints on faint galaxy redshifts from gravitational lensing studies, which imply that the bulk of the r < 26 field galaxies should be at redshifts of order 1. Our derived Aw is significantly lower than that predicted from the local bright galaxy correlation function using the lensing-determined galaxy redshift distribution and modest growth of clustering. This simplistic model does not include the variation in observed morphological mix as a function of redshift and apparent magnitude in our sample. At our faintest limits we reach sufficiently high redshifts that differential K corrections will result in the observed galaxy mix being dominated by star bursting dwarf and low surface brightness irregulars, rather than the early-type systems used to define the local bright galaxy correlation function. Adopting the correlation function measured locally for these low surface brightness galaxies and assuming modest clustering evolution, we obtain reasonable agreement between our model and observations. This model supports the scenario in which the high number density of faint galaxies is produced by normally clustered star forming dwarf galaxies at modest redshifts.

Using a non-parametric procedure developed by Kaiser & Squires (1993), we analyse the statistical image distortions of faint field galaxies to I~25.5 in two distant X-ray luminous clusters 1455+22 (z=0.26) and 0016+16 (z=0.55) to derive two dimensional projected mass distributions for the clusters. The mass maps of 1455+22 and 0016+16 are presented at effective resolutions of 135 kpc and 200 kpc respectively (for h=0.5, q_o=0.5) with a mean signal to noise per resolution element of 17 and 14. We compare our 2-D mass distributions on scales up to ~1 Mpc with those defined by the spatial distribution of colour-selected cluster members and from deep high resolution X-ray images of the hot intracluster gas. Despite the different cluster morphologies, one being cD-dominated and the other not, in both cases the form of the mass distribution derived from the lensing signal is strikingly similar to that traced by both the cluster galaxies and the hot X-ray gas. We find some evidence for a greater central concentration of dark matter with respect to the galaxies. The overall similarity between the distribution of total mass and that defined by the baryonic components presents a significant new observational constraint on the nature of dark matter and the evolutionary history of rich clusters.

Deep Hubble Space Telescope images of superlative resolution obtained for the distant rich cluster AC114 (z=0.31) reveal a variety of gravitational lensing phenomena for which ground-based spectroscopy is available. We present a luminous arc which is clearly resolved by HST and appears to be a lensed z=0.64 sub-L star spiral galaxy with a detected rotation curve. Of greatest interest is a remarkably symmetrical pair of compact blue images separated by 10 arcsec and lying close to the cluster cD. We propose that these images arise from a single very faint background source gravitationally lensed by the cluster core. Deep ground-based spectroscopy confirms the lensing hypothesis and suggests the source is a compact star forming system at a redshift z=1.86. Taking advantage of the resolved structure around each image and their very blue colours, we have identified a candidate third image of the same source roughly 50 arcsec away. The angular separation of the three images is much larger than previous multiply-imaged systems and indicates a deep gravitational potential in the cluster centre. Resolved multiply-imaged systems, readily recognised with HST, promise to provide unique constraints on the mass distribution in the cores of intermediate redshift clusters.

We present deep galaxy counts in the K (lambda 2.2um) band, obtained with the W.M. Keck 10m telescope. The data reach limiting magnitudes K~24 mag, about 5 times deeper than the deepest published K-band images to date. The counts are performed in three small (~1') widely separated high-latitude fields. Extensive Monte Carlo tests were used to derive the completeness corrections and minimize photometric biases. The counts continue to rise, with no sign of a turnover, down to the limits of our data, with logarithmic slope of dN/dm = 0.315+/-0.020 between K=20 and 24 mag. This implies a cumulative surface density of ~ 5x10^5 galaxies per sq. degree, or ~2x10^10 over the entire sky, down to K=24 mag. Our counts are in good agreement with, although slightly lower than, those from the Hawaii Deep Survey by Cowie and collaborators; the discrepancies may be due to small differences in the aperture corrections. The observed field-to-field variations are as expected from Poisson noise and galaxy clustering as described by the angular two-point correlation function for faint galaxies. We compare out counts with some of the available theoretical predictions. The data do not require models with a high value of Omega_o, but can be well fitted by models with no (or little) evolution, and cosmologies with a low value of Omega_o. Given the uncertainties in the models, it may be premature to put useful constraints on Omega_o from the counts alone. Optical-to-IR colors are computed, using CCD material previously obtained at Palomar. We find a few red galaxies with (r-K) > 5 mag or (i-K) > 5 mag; these may be ellipticals at z~1. While the redshift distribution of galaxies in our counts is still unknown, the flux limits reached would allow us to detect unobscured L* galaxies out to substantial redshifts (z > 3?).

We have completed a feasibility study for the measurement of weak distortion of distant galaxy images by intervening large-scale structure by using the 5-m Hale reflector to acquire a very deep, r~26, exposure of a single field. The error budget of our observations is dominated by the effects of atmospheric seeing (which strongly degrades this signal because the faintest images are under-resolved) and telescope effects. After performing a correction for telescope aberrations and possible guiding errors, the observed mean `polarisation' of the images of 4363 galaxies with magnitudes 23 < r < 26 within a circle of radius 4.8 arcmin was found to be p=0.01+/-0.01. The associated two-point polarisation correlation function has a constant value of C_pp = (1.4+/-3.0)x10^-5 over the angular range 1 to 6 arcmin. It is predicted that the cosmological polarisation should be in the range p = 0.03+/-0.01 for a standard CDM universe normalized with a bias parameter, b, of unity (p scales inversely with b and approximately linearly with Omega_o). For the atmospheric seeing and sky noise conditions associated with our observations, Monte Carlo simulations suggest that the efficiency of measuring the mean cosmological polarisation is of order of 40+\-10 per cent. Thus our preliminary analysis suggests an upper limit on the cosmological mean polarisation in the field of p_max~0.04. Deep wide-field imaging in 0.5 arcsec seeing to study the polarisation signal should provide limits that constrain current cosmological models.

From deep optical images of three clusters selected by virtue of their X-ray luminosity and/or optical richness (1455+22; z=0.26, 0016+16; z=0.55 and 1603+43; z=0.89), we construct statistically-complete samples of faint field galaxies (I~25) suitable for probing the effects of gravitational lensing. By selecting clusters across a wide redshift range we separate the effects of the mean redshift distribution of the faint field population well beyond spectroscopic limits and the distribution of dark matter in the lensing clusters. A significant lensing signature is seen in the two lower redshift clusters whose X-ray properties are well-constrained. Based on these and dynamical data, it is straightforward to rule out field redshift distributions for I=25 which have a significant low redshift excess compared to the no evolution prediction, such as would be expected if the number counts at faint limits were dominated by low-z dwarf systems. The degree to which we can constrain any high redshift tail to the no evolution redshift distribution depends on the distribution of dark matter in the most distant lensing cluster. In the second paper in this series, we use the lensing signal to reconstruct the full two-dimensional mass distribution in the clusters and, together with high resolution X-ray images, demonstrate that their structural properties are well-understood. The principal result is therefore the absence of a dominant low-z dwarf population to I=25.

We present the first results of an ongoing program we are undertaking with the Wide Field Camera of the Hubble Space Telescope (HST) to understand the physical origin of the enhanced star formation seen in moderate redshift z~0.3-0.4) cluster galaxies. Deep HST exposures have been obtained for the central regions of two rich compact `Butcher-Oemler' clusters, AC114 at z=0.31 and Abell 370 at z=0.37. Both of these clusters have been subject to extensive ground-based spectroscopic and multi-band imaging studies which enable us to identify, on an individual basis, members involved in various stages of starburst activity. We have used the HST images, in conjunction with the ground-based data, to examine the morphological nature of cluster members according to these stages. We find that those blue members that display spectral evidence of active or recently completed star formation are predominantly disk-dominated systems, whose abundance is greater than that seen in present-day rich clusters. There is convincing evidence in these first two clusters we have examined that interactions and mergers play a major role in inciting the high star formation activity associated with the Butcher-Oemler effect. We demonstrate the unique role HST can play in distinguishing between dynamical interactions and irregular features such as HII regions. Of equal significance is the morphological nature of the numerous red members in our HST data which show various spectroscopic and photometric indications of recent star formation, including strong Balmer absorption lines (the `E+A' phenomenon). Most of these galaxies appear to be undisturbed and isolated with a normal E morphology. There is no convincing evidence that these are merger products. Although larger samples are still required, we conjecture that the Butcher-Oemler effect may involve at least two physical processes : galaxy-galaxy interactions and environmentally-induced star formation arising from the hierarchical merging of clusters. Much work remains to be done to understand exactly how the fraction of disk galaxies seen in distant rich clusters declines to its present low value.

We present near-infrared photometry of all available gravitationally lensed `arcs' with spectroscopic redshifts. By combining this photometry with optical data, we find that the bulk of the systems with z~1 are intrinsically blue across the rest-frame spectral region 2000A to 1um. Using a combination of optical and optical-infrared colours we demonstrate that these systems cannot be blue by virtue of a secondary burst of star formation superimposed on an evolved population, but we are unable to directly distinguish between major star formation events in a generic young galaxy and an extended era of constant star formation typical of late-type spirals. Using various arguments we conclude that our arcs represent modest gravitational magnification of typical field galaxies. Consequently, if the star formation seen is representative of that in field galaxies at z > 1 , the absence of high redshift galaxies in current deep spectroscopic surveys to B~24 supports the hypothesis in which the bulk of the star formation in normal galaxies occurred over an extended era up to the epoch corresponding to z~1.

Using data from an earlier search for supernovae in distant clusters, we have constructed a homogeneous set of V images for 19 rich clusters of mean redshift z=0.32. By considering the images above a fixed surface brightness limit we have analysed the data for extended arc-like features which might arise from gravitational lensing of background sources. A list of 20 candidate arcs is presented. We examine the usefulness of such a catalogue for deriving the background source redshift distribution, N(z). Whilst the number and shape distribution of arc candidates is consistent with the lensing hypothesis, cluster velocity dispersions of very high precision would be needed to provide useful constraints on the fraction of high-redshift galaxies to faint limits. We show how, in principle, a likelihood ratio test based on the radial distribution of arcs in a single well-studied cluster could determine whether a significant fraction of the faint field population is at high redshift. This test also provides a means of determining accurate cluster velocity dispersions at any redshift.

Spectra are presented for portions of the two arcs observed close to the dominant cD galaxy in the rich cluster Abell 963 (z=0.206). The spectrum of the northern arc displays a strong emission line at 6600A which is seen along the entire arc. The feature cannot be understood unless the redshift is greater than that of the cluster, the most likely interpretation being [OII] 3727A at z=0.771. The southern arc is considerably fainter and its spectrum shows no obvious features. However, new CCD photometry is consistent with a near-constant blue colour (B-R~0.3) along both arcs supporting the suggestion that they arise from the gravitationally lensed light of a background object. The optical and infrared colour is consistent with a spiral galaxy undergoing strong star formation at this redshift. We discuss briefly the implication of this result, and the possible role lensing surveys may play in the study of high redshift galaxies.