ON THE PECULIAR VELOCITY-FIELD OF A CDM UNIVERSE

Using a large N-body simulation of a standard cold dark matter univers e in which individual dark matter galaxy halos can be resolved, the pe culiar velocity field of the halos and mass particles is investigated. The rms velocity (v(rms)), velocity correlation function [xi(vv)(r)], and the three-dimensional pairwise velocity dispersion (sigma(vv)) of the halos and mass are determined. The effects of halo mass and local environment on these functions are investigated. High-mass halos with overdensities of similar to 250 and similar to 70 are good tracers of the major mass motions in the simulation as defined by a volume-avera ged velocity field. Low-mass halos are biased tracers of the same fiel d. Assuming one luminous galaxy would reside in each of the halos and the mass-to-light ratio is a constant, this implies bright galaxies ar e fair tracers of the major mass motions in a CDM universe. The rms ve locity of the halos is strongly affected by the local environment; the higher the background mass density the larger v(rms). However, it is not straightforward to determine the magnitude of an enhancement/suppr ession in the local mass density given the local enhancement/suppressi on of v(rms). At the end of the simulation, we obtain values of the ve locity bias, b(v) = sigma(vv,h)/sigma(vv,m), as a function of halo mas s and minimum overdensity similar to those found by Carlberg, Couchman , and Thomas and by Carlberg. For halos with overdensities of similar to 250 and similar to 70, b(v) is a decreasing function of background mass density (i.e., the higher the background density, the larger the discrepancy between the velocity dispersions of the halos and mass par ticles), and for halos with overdensities of similar to 2000 it is an increasing function of background density (i.e., the lower the backgro und density, the larger the discrepancy between the velocity dispersio ns of the halos and mass particles). The velocity bias as a function o f scale, b(v)(r), is an increasing function of separation and even on large scales, r similar to 1400 km s(-1), a noticeable velocity bias i s present at the end of the simulation (similar to 0.9 for halos with overdensities similar to 2000, similar to 0.8 for halos with overdensi ties similar to 250, and similar to 0.7 for halos with overdensities - 70). Little difference is found between xi(vv)(r) for halos and mass p articles, and xi(vv)(r) is similar for halos of high and low mass. Mak ing the assumption that one luminous galaxy would reside in each of th e halos and the mass-to-light ratio is a constant, this suggests the v elocity correlation statistics of both faint and bright galaxies are r epresentative of the velocity correlation statistics of the mass in a CDM universe. By the end of the simulation, xi(vv)(r) is about 70% of the value predicted by linear theory over all scales investigated. Lit tle environmental effect on xi(vv)(r) of halos with overdensities of s imilar to 2000 is observed. On scales less than or similar to 630 km s (-1), the power in xi(vv)(r) for halos with overdensities of similar t o 200 and similar to 70 is due to halos in regions of high background mass density.