INTERNAL KINEMATICS OF GROUPS OF GALAXIES IN THE SLOAN DIGITAL SKY SURVEY DATA RELEASE 7

We present measurements of the velocity dispersion profile (VDP) for galaxy groups in the final data release of the Sloan Digital Sky Survey (SDSS). For groups of given mass, we estimate the redshift-space cross-correlation function (CCF) with respect to a reference galaxy sample, ξ(s)(rp, π), the projected CCF, wp(rp), and the real-space CCF, ξcg(r). The VDP is then extracted from the redshift distortion in ξ(s)(rp, π), by comparing ξ(s)(rp, π) with ξcg(r). We find that the velocity dispersion (VD) within virial radius (R200) shows a roughly flat profile, with a slight increase at radii below ∼0.3R200 for high-mass systems. The average VD within the virial radius, σv, is a strongly increasing function of central galaxy mass. We apply the same methodology to N-body simulations with the concordance Λ cold dark matter cosmology but different values of the density fluctuation parameter σ8, and we compare the results to the SDSS results. We show that the σv − M* relation from the data provides stringent constraints on both σ8 and σms, the dispersion in log M* of central galaxies at fixed halo mass. Our best-fitting model suggests σ8 = 0.86 ± 0.03 and σms = 0.16 ± 0.03. The slightly higher value of σ8 compared to the WMAP7 result might be due to a smaller matter density parameter assumed in our simulations. Our VD measurements also provide a direct measure of the dark matter halo mass for central galaxies of different luminosities and masses, in good agreement with the results obtained by Mandelbaum et al. from stacking the gravitational lensing signals of the SDSS galaxies.