  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, Spitzer photometry and
ground-based spectroscopy cover most of a region of the cluster about 10 arcmin
across, corresponding to a clustercentric 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 starforming galaxies in A851 have 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.
