The Observational Distribution of Internal Velocity Dispersions in Nearby Galaxy Clusters

Abstract

We analyze the internal velocity dispersions of a sample of 172 nearby galaxy clusters (z < 0.15), each of which has at least 30 available galaxy redshifts, and spans a large richness range. Cluster membership selection is based on nonparametric methods. In the estimate of galaxy velocity dispersion we consider the effects of possible velocity anisotropies in galaxy orbits, the infall of late-type galaxies, and velocity gradients. The dynamical uncertainties due to the presence of substructures are also taken into account. Our cumulative distribution agrees with previous distributions within their completness in velocity dispersion ( > 800 km/s). We estimate that our distributions is complete for at least a dispersion greater than 650 km/s. In this completeness range a power law fit is in fair agreement with results coming from the X-ray temperature distributions.