The Dynamics of Small Groups of Galaxies. I. Virialized Groups

A numerical code is developed to follow the dynamical evolution of groups of galaxies, starting from virial equilibrium. The code assigns a single particle to each galaxy and to a diffuse intergalactic background, both with appropriately softened potentials, and explicitly incorporates many of the physical processes occuring in groups, such as collisional stripping, tidal stripping from the background mean field, dynamical friction on the background, mergers, and orbital braking. Groups of eight galaxies with surface densities similar to Hickson's compact groups are unstable against rapid galaxy merging. No dense groups of eight galaxies still appear compact (in Hickson's sense) after 1/2t_Hubble. These "instability" times are insensitive to the uncertainties of the physics used in the code, but are increased with larger group membership, and larger group mass-luminosity ratios.