The Dependence of the Pairwise Velocity Dispersion on Galaxy Properties

We present measurements of the pairwise velocity dispersion (PVD) for different classes of galaxies in the Sloan Digital Sky Survey (SDSS). For a sample of about 200,000 galaxies with redshifts in the interval 0.01 < z < 0.3, and r-band magnitudes M0.1 r between −16 and −23, we study the dependence of the PVD on galaxy properties such as luminosity, stellar mass (M∗), colour (g− r), 4000Å break strength (D4000), concentration index (C), and stellar surface mass density (μ∗). The luminosity dependence of the PVD is in good agreement with the results of Jing & Börner (2004) for the 2dFGRS catalog, once the photometric bandpass difference between the two surveys is taken into account. The value of σ12 measured at k = 1 h Mpc −1 decreases as a function of increasing galaxy luminosity for galaxies fainter than L∗, before increasing again for the most luminous galaxies in our sample. Each of the galaxy subsamples selected according to luminosity or stellar mass is divided into two further subsamples according to colour, D4000, C and μ∗. We find that galaxies with redder colours and higher D4000, C, and μ∗ values have larger PVDs on all scales and at all luminosities/stellar masses. The dependence of the PVD on parameters related to recent star formation (colour, D4000) is stronger than on parameters related to galaxy structure (C, μ∗), especially on small scales and for faint galaxies. The reddest galaxies and galaxies with high surface mass densities and intermediate concentrations have the highest pairwise peculiar velocities, i.e. these move in the strongest gravitational fields. We conclude that the faint red population located in rich clusters is responsible for the high PVD values that are measured for low-luminosity galaxies on small scales.