AAS 197, January 2001
Session 57. Galaxy Clusters: Galaxy Evolution
Oral, Tuesday, January 9, 2001, 10:30am-12:00noon, San Diego

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[57.04] The Radio Butcher-Oemler Effect

G. E. Morrison (Caltech-IPAC and Vanguard Research Inc.)

The purpose of the dissertation was to investigate the population of low-luminosity radio galaxies within very-rich clusters of galaxies, from the dense cores out to low density regions 2.5 Mpc from the cluster centers, and over a significant interval of cosmological time, ~5 Gyr. Based on the local radio luminosity function and the far-infrared radio correlation, many low-luminosity radio galaxies have their radio emission powered by synchrotron radiation resulting from ongoing star-formation.

A VLA 20 cm radio continuum survey was performed, observing 34 very-rich clusters with 0.02 < z < 0.41, down to a constant radio luminosity limit of 2 x 1022 W Hz-1. It was found that the fraction of radio-selected galaxies in distant rich z ~0.4 clusters with luminosities in the range 1022.3 W Hz-1 < L1.4 < 1022.75 W Hz-1 is higher by a factor of > 6x compared to nearby rich clusters. At least 15% of the galaxy population in the distant clusters with MR > -21 are low-luminosity radio galaxies, compared to ~2% for local rich clusters. Spectroscopic results suggest that this population is made up of star-forming galaxies. The X-ray and optically inferred substructure of some clusters suggests that ongoing cluster-cluster mergers may increase this population regardless of redshift.

A significant discovery was made in regards to the `E+A' population of the distant (z = 0.4) rich cluster Cl0939+4713. These optically classified `post-starburst' galaxies, previously believed to have been quiescent for the last ~1 Gyr, have radio emission consistent with ongoing star-formation in excess of 5 MO yr-1. Extinction maps constructed from HST NICMOS and WFPC2 images of several radio-selected galaxies in Cl0939+4713 suggest that significant quantities of dust are located in these galaxies, which could completely hide star-formation inferred from optical techniques. The radio-selected galaxies are associated with two different populations of cluster galaxies: AGNs (high-luminosity radio emission) in early-type galaxies and dusty starbursting (low-luminosity radio emission) late-type spirals.

Financial support was provided by the NRAO predoctoral fellowship program and NASA.

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