Relaxation and tidal stripping in rich clusters of galaxies. II - Evolution of the luminosity distribution

Abstract

Two sorts of arguments are presented which suggest that the luminosity distribution in rich clusters of galaxies-including the morphology of first-ranked or cD galaxies-is fixed during cluster collapse and changes very little afterward. First, it is shown that some of the properties of bright cluster galaxies that are often taken to imply ongoing evolution, such as their central locations and extended envelopes, are more likely a result of dynamical processes that occurred during cluster collapse. Second, it is shown that truncation of galaxy halos during cluster collapse is probably sufficiently strong to 'turn off' subsequent evolution by making time scales for dynamical friction and collisional stripping longer than a Hubble time. The dynamical evolution of a cluster after its formation is simulated using the computer code described in an earlier paper, after a modification which permits galaxy masses and luminosities to be treated independently. The fraction of a cluster's binding matter that is attached to galaxies is assumed to be set by the limiting effect of the cluster's mean tidal field. The results suggest that the number of merger candidates in the core of a rich cluster does not increase significantly as a result of dynamical evolution. Contrary to recent claims by various authors, the growth rate of a cD galaxy due to mergers should therefore be negligible, unless a substantial population of galaxies is bound to the cD at cluster formation.