Glenn Morrison, NOAO Tucson

	Multiwavelength observations of a very rich cluster, CL0939+4713


Multiwavelength observations ability to sample a large fraction of a
galaxy's SED provides us with valuable data that is required to fully
assess the ongoing physical processes in these systems. To this end,
sensitive radio observations provide a unique perspective on the
evolution of galaxies by providing an unobstructed view of ongoing
star-formation and/or AGN activity. My talk will present results from
a radio/optical/NIR/X-ray study of the very-rich cluster Cl0939+4713 at
z=0.41.  This cluster has the largest fraction (32%) of radio-selected
galaxies (L[1.4GHz] > 2E22 W/Hz) out of a sample of 36 very-rich
clusters (0.02<z<0.41) which have an average radio-selected galaxy
fraction of 2.0+\-0.3%. In Cl0939+4713, ~70% of radio-selected
galaxies are disturbed late-type spirals with post-starburst, dusty
starburst, or optical starburst spectroscopic signatures. In fact,
CL0939+4713 was the first environment in which poststarburst galaxies
were found to have significant radio emission. The fraction of
low-luminosity radio galaxies within these very-rich clusters has
decreased by a factor ~6-10 over the last 5 Gyr. The physical
mechanism for the increased fraction of radio-selected galaxies in
CL0939+4713 seems to be predominantly galaxy-galaxy mergers with a
strong indication that substructure tidal effects and large scale
structure buildup, e.g., cluster-cluster mergers might also be affecting
this cluster population.

Wednesday January 19th, at 11am