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E. Ellingson (University of Colorado), R. Abraham, H. K. C. Yee (University of Toronto), H. Lin (University of Arizona), G. Laurent (University of Colorado), D. Schade (Dominion Astrophysical Observatory)
We present results from extensive photometric, spectroscopic and morphological observations of a sample of rich galaxy clusters at 0.17 < z < 0.6. Our program has been designed to explore gradients in cluster populations to radii of 1-3 Mpc in the redshift range where the Butcher-Oemler effect is strongly observed. We find spatial gradients in the spectroscopic and morphological properties of cluster galaxies as a function of cluster-centric redshift, and model them in terms of an older, evolved population and an infalling population with the same properties as coeval field galaxies. After their entry into the cluster, star formation in these galaxies is terminated by the cluster environment. We find that the observed evolution in the galaxy population is consistent with a scenario where the Butcher-Oemler effect is due to a decline in the infall rates into clusters declines over time. We then test simple disk-fading models for the morphological transformation of the infalling field galaxies to the evolved cluster population.