Description
We use extensive new observations of the very rich z~0.4 cluster of galaxies A851 (Cl 0939+47) to examine the nature and origin of starburst galaxies in intermediate-redshift clusters. New HST observations, 24um 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 1.6Mpc. 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.
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