THE SMALL-SCALE VELOCITY DISPERSION OF GALAXIES - A COMPARISON OF COSMOLOGICAL SIMULATIONS

The velocity dispersion of galaxies on small scales (r similar to 1 h( -1) Mpc), sigma(12)(r), can be estimated from the anisotropy of the ga laxy-galaxy correlation function in redshift space (Davis & Peebles). We apply this technique to ''mock catalogs'' extracted from N-body sim ulations of several different variants of cold dark matter-dominated c osmological models, including models with cold + hot dark matter, to o btain results that may be compared consistently with similar results f rom observations. We find a large variation in the value of sigma(12)( 1 h(-1) Mpc) in different regions of the same simulation. We investiga te the effects of removing clusters from the simulations, using an aut omated cluster-removing routine, and find that this reduces the sky va riance but also reduces the discrimination between models. However, st udying sigma(12) as clusters with different internal velocity dispersi ons are removed leads to interesting information about the amount of p ower on cluster and subcluster scales. We compute the pairwise velocit y dispersion directly, in order to check the Davis-Peebles method, and find agreement of better than 20% in all the models studied. We also calculate the mean streaming velocity and the pairwise peculiar veloci ty distribution in the simulations, and compare these with the models used in the Davis-Peebles method. We find that the model for the mean streaming velocity may be a substantial source of error in the calcula tion of sigma(12).