Virial properties of groups of galaxies

Abstract

Modern techniques and data are used to derive virial parameters for numerous well-known groups of galaxies. Virial mass-to-light ratios are typically 40-120 solar units. The groups are gravitationally bound because (a) their crossing times are smaller than a Hubble time, and (b) the composite frequency distribution of their radial velocities is 'approximately' Gaussian. A correlation is observed between the virial mass-to-light ratio and the virial velocity dispersion. Contributions to this correlation by projection and quasi-equilibrium effects, radial velocity uncertainties, and uncertainties in galactic mass are evaluated and found to be too small to explain the correlation. The average surface density of galaxies in groups appears to be independent of distance from the center out to a mean limiting radius.