ABELL 851 AND THE ROLE OF STARBURSTS IN CLUSTER GALAXY EVOLUTION

We use extensive new observations of the very rich z ∼ 0.4 cluster of galaxies A851 to examine the nature and origin of starburst galaxies in intermediate-redshift clusters. New HST observations, 24 μm Spitzer photometry and ground-based spectroscopy cover most of a region of the cluster about 10′ across, corresponding to a cluster-centric radial distance of about 1.6 Mpc. This spatial coverage allows us to confirm the existence of a morphology–density relation within this cluster, and to identify several large, presumably infalling, subsystems. We confirm our previous conclusion that a very large fraction of the star-forming galaxies in A851 has recently undergone starbursts. We argue that starbursts are mostly confined to two kinds of sites: infalling groups and the cluster center. At the cluster center, it appears that infalling galaxies are undergoing major mergers, resulting in starbursts whose optical-emission lines are completely buried beneath dust. The aftermath of this process appears to be proto-S0 galaxies devoid of star formation. In contrast, major mergers do not appear to be the cause of most of the starbursts in infalling groups, and fewer of these events result in the transformation of the galaxy into an S0. Some recent theoretical work provides possible explanations for these two distinct processes, but it is not clear whether they can operate with the very high efficiency needed to account for the very large starburst rate observed.