INDIVIDUAL AND GROUP GALAXIES IN CNOC1 CLUSTERS

Using wide-field BVRcI imaging for a sample of 16 intermediate redshift (0.17 < z < 0.55) galaxy clusters from the Canadian Network for Observational Cosmology Survey, we investigate the dependence of cluster galaxy populations and their evolution on environment. Galaxy photometric redshifts are estimated using an empirical photometric redshift technique and galaxy groups are identified using a modified friends-of-friends algorithm in photometric redshift space. We utilize the red galaxy fraction (f(red)) to infer the evolutionary status of galaxies in clusters, using both individual galaxies and galaxies in groups. We apply the local galaxy density, Sigma(5), derived using the fifth nearest neighbor distance, as a measure of local environment, and the cluster-centric radius, r(CL), as a proxy for global cluster environment. Our cluster sample exhibits a Butcher-Oemler effect in both luminosity-selected and stellar-mass-selected samples. We find that f(red) depends strongly on Sigma(5) and r(CL), and the Butcher-Oemler effect is observed in all Sigma(5) and r(CL) bins. However, when the cluster galaxies are separated into r(CL) bins, or into group and nongroup subsamples, the dependence on local galaxy density becomes much weaker. This suggests that the properties of the dark matter halo in which the galaxy resides have a dominant effect on its galaxy population and evolutionary history. We find that our data are consistent with the scenario that cluster galaxies situated in successively richer groups (i.e., more massive dark matter halos) reach a high f(red) value at earlier redshifts. Associated with this, we observe a clear signature of preprocessing, in which cluster galaxies belonging to moderately massive infalling galaxy groups show a much stronger evolution in f(red) than those classified as nongroup galaxies, especially at the outskirts of the cluster. This result suggests that galaxies in groups infalling into clusters are significant contributors to the Butcher-Oemler effect.