On the streaming motions of haloes and galaxies

A simple model is described of how objects of different masses stream towards each other as they cluster gravitationally. The model shows how the mean streaming velocity of dark matter particles is related to the motions of the parent dark matter haloes. It also provides a reasonably accurate description of how the pairwise velocity dispersion of dark matter particles differs from that of the parent haloes. The analysis is then extended to describe the streaming motions of galaxies. This shows explicitly that the streaming motions measured in a given galaxy sample depend on how the sample was selected, and shows how to account for this dependence on sample selection. In addition, we show that the pairwise dispersion should also depend on sample type. Our model predicts that, on small scales, redshift-space distortions should affect red galaxies more strongly than blue.