On the luminosity dependence of the galaxy pairwise velocity dispersion

We make predictions for the pairwise velocity dispersion (PVD) of galaxies with models that are constrained to match the projected correlation function and luminosity function of galaxies in the Two Degree Field Galaxy Redshift Survey (2dFGRS). We use these data to constrain the halo occupation distribution (HOD) of 2dFGRS galaxies, then calculate the PVD by populating the halos of a large-volume, high-resolution N-body simulation. We examine the luminosity and scale dependence of the predicted PVD. At small and large scales, r < 1 and r greater than or similar to 3 h(-1) Mpc, we find that the PVD decreases with increasing galaxy luminosity. This result is mostly driven by the fraction of satellite galaxies f(sat), which is well constrained by the correlation function. We find f(sat) similar to 25% for galaxies fainter than L(*), while for brighter galaxies the satellite fraction rapidly declines, creating the decrease in the PVD with luminosity. Recent measurements of the PVD in Fourier space using the dispersion model'' have reported a strong decline in the PVD with increasing luminosity at k = 1 Mpc(-1) h. We test this method with our HOD models, finding a luminosity dependence in k-space that is stronger than that in configuration space at that scale. The luminosity dependence of the HOD results in Fourier space are consistent with those measured at k = 1 Mpc(-1) h; thus, the recent measurements of the PVD are fully explainable in the context of halo occupation models. The normalization of the PVD is lower than that predicted by our fiducial model, and reproducing it requires a lower value of Omega(m) (similar to 0.2 instead of 0.3), a lower value of sigma(8) (similar to 0.7 instead of 0.9), or a significant bias between the velocity dispersion of galaxies and dark matter in massive halos.