The Pairwise Velocity Dispersion of Galaxies: Luminosity Dependence and a New Test of Galaxy Formation Models

We present the first determination of the pairwise velocity dispersion (PVD) for galaxies in different luminosity intervals using the final release of the Two-Degree Field Galaxy Redshift Survey (2dFGRS). We have discovered quite surprisingly that the relative velocities of the faint galaxies at small separation are very high, around 700 km s-1, reaching similar values as the brightest galaxies. At intermediate luminosities M* - 1 (M* is the characteristic luminosity of the Schechter function), the relative velocities exhibit a well-defined steep minimum near 400 km s-1. This result has been derived using a novel method to determine the real-space power spectrum and the PVD from the redshift-space power spectrum of the 2dFGRS. Combined with the observed luminosity dependence of clustering, our result implies that quite a fraction of faint galaxies, as well as the brightest ones, are in massive halos of galaxy cluster size, but most of the M* galaxies are in galactic halos. Our observed result is compared with the current halo model of galaxies of Yang et al., which was obtained by matching the clustering and luminosity functions of the 2dFGRS. With the model parameters they favored most, the halo model seems to be unable to reproduce the luminosity dependence of the PVD, because it predicts a monotonically increasing PVD with the luminosity. We discuss a possible solution to this model by raising the faint-end slope of the conditional luminosity function in rich clusters. The PVD luminosity dependence may also be an important constraint in general on theories of galaxy formation, such as semianalytical models and hydro/N-body simulations of galaxy formation.