Choice of Observer

1. Observers' velocity. We impose the amplitude of the observers' peculiar velocity to be in the range v_LG = 625 +/- 25 km/s to match the observed Cosmic Microwave Background [CMB] dipole (Lineweaver et al. 1996). The orientation of the coordinate system is then chosen to that this points towards $(l,b)=(276,30)$ to match the direction of the CMB dipole.

   Orienting the frame in this fashion is aimed at mimicking the dipole moment of the mass distribution in the real universe and has the usual effect of placing a large overdensities at the positions in which we actually observe the Great Attractor and, in some cases, a second density peak in correspondence of the Perseus Pisces superclusters (Branchini et al. 1999).

2. Amplitude of the bulk flow at 30 Mpc/h. The amplitude of the bulk velocity measured at 30 Mpc/h, v_30, is chosen to be in the range 400--480 km/s to match the value obtained from the Mark III--POTENT analysis performed by (Dekel et al. 1998). Note that the smoothing procedure performed in the POTENT analysis allows the authors to perform a volume weighted bulk flow estimate, fully consistent with the one obtained from our analysis of N-body data, in which the bulk flow is computed after smoothing the velocity field onto a regular cubic grid with a mesh of 5 Mpc/h.
3. Direction of the bulk flow at 30 Mpc/h. As a third constraint we select only those observers for which the misalignment between v_30 and v_L, is smaller than 40 degreses. T his observational upper limit is also obtained from the work of Dekel et al (1998).
4. Outflow at 70 Mpc/h . This constraint is derived from the recent work of Zehavi et al. (1998) in which the authors use 44 Type 1a supernovae and estimate the fractional deviation from a pure Hubble flow inside a sphere of radius 70 Mpc/h to be 6.6+/-2.2 %.