Text: A team of astronomers from England and France have reported strong evidence for the existence of a rare type of black hole, called a Type 2 quasar. Using NASA's Chandra X-ray Observatory, they have discovered a powerful source of X rays that appears to be a giant black hole that is hidden from optical telescopes by a veil of obscuring material.
The latest discovery comes from a team led by British astronomers Andrew Fabian of the Institute of Astronomy in Cambridge and Ian Smail of the University of Durham. They used Chandra, NASA's Hubble Space Telescope, and the James Clerk Maxwell submillimeter telescope on Mauna Kea in Hawaii. By concentrating their search near two galaxy clusters, the astronomers exploited a gravitational lensing effect that can lead to a significant brightening of distant sources.
Four new X-ray sources and seven submillimeter sources were discovered. The brightest X-ray source is concentrated in the center of a distant galaxy. The point-like source has a deficit of low energy X rays, consistent with absorption by a thick cloud of gas. The combination of powerful X-ray emission, absorption of low energy X rays, and the relatively normal optical appearance of the galaxy led the scientists to conclude that the source is a strong contender to be a genuine Type 2 quasar.
Type 2 quasars have been predicted to exist by a popular model for quasars. Their discovery would confirm the so-called unified model for quasars, and help clarify the nature of the pervasive background glow at X-ray and submillimeter energies. Evidence for Type 2 quasars has been reported by other researchers, but the data were ambiguous. Now, the cloud of uncertainty is lifting, as scientists use Chandra to intensify the search.
According to the unified model of quasars, a thick doughnut of gas and dust surrounds a central black hole. The source looks different, depending on whether it is observed through the doughnut, through the hole, or at an intermediate angle. In extreme cases representing a Type 2 quasar, the optical radiation from the quasar is absorbed while the high energy X rays penetrate the veil. The energy absorbed by the gas and dust is re-radiated at lower energy infrared and submillimeter wavelengths.
Six of the submillimeter sources that were discovered were not detected in X rays. This could mean that a central black hole is shrouded by an unusually thick cloud of dust and gas, or an additional source of submillimeter radiation is present, perhaps due to a burst of star formation.
A paper describing these results will be published in the Monthly Notices of the Royal Astronomical Society. The Chandra observations were made on November 5, 1999 using the Advanced CCD Imaging Spectrometer (ACIS). The team involved scientists from the Instititute of Astronomy, the University of Durham, University College London, and the Observatoire Midi-Pyrenees in France.
The ACIS instrument was built for NASA by the Massachusetts Institute of Technology, Cambridge, and Pennsylvania State University, University Park. NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program. TRW, Inc., Redondo Beach, Calif., is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Mass.
Dr. Fabian can be reached at acf@ast.cam.ac.uk, and Dr. Smail at Ian.Smail [at] durham.ac.uk
Press release Credits: CXC PR: 00-09
Steve Roy
Marshall Space Flight Center, Huntsville, AL,
Phone: 256-544-6535
http://www.msfc.nasa.gov/news/
Dr. Wallace Tucker
Chandra X-ray Observatory Center
Harvard-Smithsonian Center for Astrophysics, Cambridge,
MA
Phone: 617-496-7998
Figure: X-ray (left) and Optical (right) Images of Veiled
Black Hole. The left hand panel shows the Chandra
X-ray Observatory image (left panel) of a powerful
point-like source of X rays. The Hubble Space
Telescope image (right panel) X-ray source is
located at the center of the galaxy, and has a
deficit shows the spiral galaxy with which the X-ray
source is associated. The of low energy X rays,
consistent with absorption by a thick cloud of gas.
The combination of powerful X-ray emission,
absorption of low energy X-rays, and the relatively
normal optical appearance of the galaxy suggests
that the source is a rare type of black hole called
a Type 2 quasar.
The spread-out appearance of the X-ray source is an
instrumental artifact. The distribution of X rays is
consistent with this source being point-like. The
X-ray and optical image panels have the same scale,
which is 10 arc-seconds on a side.
Photo credit: X-ray NASA/IoA/Fabian et al., Optical NASA/U. Durham/Smail et al.
Background text: Since the lauch of the first X-ray satellites in the early 1960's Astronomers have known of a uniform background of hard X-ray radiation coming from all over the sky. The source of this radiation has remained elusive until the launch of the Chandra X-ray observatory at the end of last year. Chandra resolved the hard X-ray background into individual sources, and working with some of the first data a Durham/Cambridge team have identified an example of one of these X-ray emitting galaxies. This relatively unassuming galaxy does not look like the typical X-ray emitting galaxy (mostly Quasars, which are usually extremely bright, or very disturbed objects). Instead the UK-team concludes that it represents a `Type-2' Quasar - (a class of galaxy which had been theoretically proposed as the origin of the hard X-ray background, but had never been seen). A Type-2 Quasar differs from the more normal Type-1 variety in that the light emitted from the central super-massive black hole (which dominates our view of normal Quasars) is absorbed by a thick shroud of dust and so the galaxy appears relatively normal - even though it harbours a powerful energy source at its core - the only indication of which are the hard X-ray which are energetic enough that they can punch through the obscuring dust and be seen by Chandra.
The work by the Durham team involved tracking down the counterpart to the Chandra sources using our Hubble Space Telescope imaging, this included visual (eye-ball) classification of the counterparts we found and computer analysis of their characteristics (e.g. colours). In contrast the Cambridge team concentrated mostly on the X-ray properties of these sources.
Below: The top two rows show the Chandra source CXOUJ215334.0+174240 as seen in hard X-rays (using two different false color maps and at two different pixel scales). The upper panels have 0.5-arcsec pixels, while the lower panels are resampled to 0.25-arcsec pixels. Both sets of panels are smoothed with a 0.8-arcsec FWHM gaussian for display purposes.
The lower three panels show the optical counterpart identified in an archival HST WFPC2 image of the field. The HST imaging is in F555W (V) and F814W (I) visual and near-infrared bands. The lower left-hand panel is a `true' color representation of the field, while the two right-hand panels show a false color view of the composite image using two different false color maps.
In all cases the panels are 10-arcsec square with North
top and East to the left. Note these images are JPEGs and the color
tables will look much better if viewed with a tool other than a browser.
Overlays of the 850um SCUBA maps onto deep CFHT/P200 I images of two clusters: Abell 370 (z=0.37) and Abell 1835 (z=0.25). The fields of view are 160 arcsec square and the submm maps show a number of significant sources in both maps. The brightest source in the A370 map has been identified as a z=2.80 starburst/agn, while one of the two bright sources in A1835 has recently been identified as a z=2.53 starburst on the basis of Keck spectroscopy undertaken by Amy Barger and Len Cowie at the University of Hawaii.
The sub-mm maps below show a number of galaxies detected at wavelengths around 850um. These objects are seen in the sub-mm because the dust within them is reprocessing UV and optical star-light and emitting large quantities of radiation in the sub-mm. Only one of these 850um sources (the brightest in the image at the upper-left, called SMM 02399-0136 or `Le Blob' for short) is seen in the image taken at wavelengths around 450um and the constraints this provides on the redshifts of the sources indicates that the majority of these objects are likely to lie at z>>1 (this is equivalent to saying we are seeing them at a time when the Universe was half its present age). If this is the case then these represent very luminous and strongly star-forming galaxies (`proto-galaxies'?). Optical surveys of the distant Universe with Hubble Space Telescope have failed to uncover this population, probably due to the high optical obscuration caused by the dust. The high surface density of the sub-mm sources indicates that the population being missed by the optical surveys could account for over half the star-formation in the distant Universe. The paper detailing these observations will appear in the November 15th edition of The Astrophysical Journal Letters (1997, 490, L5-L8) and is also available as postscript from here. A paper discussing the details properties of Le Blob has been submitted to the Monthly Notices of the Royal Astronomical Society, a preprint is available here.
CAPTION:These are SCUBA maps of two distant cluster lenses, one called A370 (top/bottom on the left) the other Cl2244-02 (on the right), and taken at two different wavelengths: 450um and 850um. There are 4 sources visible in the 850um map of A370, and the brightest of these (at the bottom of the image) has been named `Le Blob'. A spectrum of this galaxy taken with the 3.6-m Canada France Hawaii Telescope by Dr Jean-Paul Kneib and Dr Jean-Francois Le Borgne of Toulouse Observatory shows that it lies at a redshift of z=2.8, or roughly two-thirds of the way back to the Big Bang. This may well be a young galaxy caught in the act of formation. All the maps are roughly 180 arcsec in diameter - or about 1/10th the diameter of the full moon - and the major tick marks show 10 arcsec.
Text of the article in The Times on 19/11/97 is here. A more detailed piece by Damian Carrington was in The Scotsman on 20/10/97. Use of the images above or below should attribute the Royal Observatory, Edinburgh.
CAPTION:These two images show different views of the same region of sky containing the gravitational cluster-lens Abell 370. On the left we have a true colour `optical' image made up of blue, red and near-infrared images taken with the 3.6-m Canada France Hawaii Telescope by Dr Jean-Paul Kneib of Toulouse Observatory. The optical image shows a giant gravitational arc seen through the centre of the cluster of galaxies, this is a image of a background galaxy distorted and magnified by the foreground cluster lens. A number of other distorted images of visible across the frame - as well as many of the galaxies actually within the cluster which appear yellow. There are obviously are a large number of very faint galaxies visible in this image, but it is impossible to tell which are the distant, young galaxies which are crucial for studying galaxy formation and evolution. However, on the right is the submillimetre SCUBA map of the same field, although this detects only a few objects these are all likely to be the dusty, young galaxies which are need to test models of galaxy formation. Thus SCUBA's submillimetre observations are critical for understanding the evolution of galaxies.
Last Modified: March 21st, 2000 [Netscape 2.0]
Ian Smail, Ian.Smail [at] durham.ac.uk