The Velocity Field Around Groups of Galaxies

A statistical method is presented for determining the velocity field in the immediate vicinity of groups of galaxies using only positional and redshift information with the goal of studying the perturbation of the Hubble flow around groups more distant than the Local Group. The velocities are assumed to obey a Hubble-like expansion law, i.e. $V=H_{exp}R$ where the expansion rate $H_{exp}$ is to be determined. The method is applied to a large, representative group catalog and evidence is found for a sub-Hubble expansion rate within two well defined radii beyond the virial radii of the groups. This result is consistent with that of Teerikorpi et al. (2008) who found a similar expansion law around 3 nearby groups and extends it to a more representative volume of space.