The Origin of Central Cluster Galaxies

Abstract

The galaxies located at the centers of clusters of galaxies are the most luminous stellar systems in the universe, and explaining the origin of these remarkable objects is a major challenge for any comprehensive theory of galaxy formation. One possibility is that central galaxies are made by a series of mergers of cluster galaxies that have spiraled to the bottom of the cluster potential well through dynamical friction. However, the resulting rate of accumulation of luminosity at the cluster center appears to be too slow—by more than a factor of two—to produce central galaxies with the luminosities observed, at least in virialized clusters similar to those we see today. A related possibility is that central galaxies are formed before the cluster virializes, during the early stages of hierarchical clustering when the relative velocities of the galaxies are low enough that merging can occur in pair wise galaxy encounters. However, so far there are no convincing numerical models of hierarchical clustering in which the most massive stellar systems formed resemble observed central galaxies. The evidence for recent merging in central galaxies is still difficult to interpret: multiple nuclei in central galaxies do not provide direct evidence for merging, as most are simply projected cluster members, but there is strong circumstantial evidence that dumbbell galaxies are the ex-central galaxies from two recently merged clusters that will themselves merge in about 0.2 Hubble times.