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Photometric Properties of Galaxy Populations in Group and Cluster Environments

I-hui Li

Doctor of Philosophy 2007
Graduate Department of Astronomy and Astrophysics, University of Toronto

Using four-band photometric redshifts we explore photometric properties of galaxy populations in group and cluster environments at 0.20 <= z < 0.60. We probe red galaxy fractions (fred) using samples of 76 galaxy clusters and ~ 200 galaxy groups from CNOC1 and RCS follow-up observations.

Within all environments where both local galaxy density and cluster-centric radius are controlled, we observe galaxy down-sizing in star formation, as evidenced by the larger fred possessed by bright galaxies than that by the faint ones. We find that the Butcher-Oemler effect is stronger within the cluster virial radius, where star formation is truncated more rapidly compared with the outskirts. Both bright and faint galaxies exhibit weak dependence of their fred on local galaxy density beyond the cluster virial radius, indicating accelerated evolution driven by galaxy clusters. The gradients between fred and local galaxy densities are larger for the bright galaxies at z ~ 0.50, but become similar for the bright and faint galaxies at z ~ 0.30. The cluster environmental influence is effective within ~ 1.5 virial radii and is stronger in high galaxy density regions, indicating that mechanisms such as harassment and tidal interactions are operating in high galaxy density locations, while ram-pressure is likely the main mechanism in regions of low galaxy density.

For environments of similar local galaxy densities and at a fixed cluster-centric radius, galaxies in groups possess larger fred than non-group galaxies, which is usually referred to as `pre-processing'. We find that the pre-processing is independent of local galaxy density and global cluster environmental impact. Instead, the group environmental influence depends on group richness. We observe a `group down-sizing' effect such that galaxies in rich groups (of more massive haloes) have their fred evolving with redshift more rapidly than those in poor groups (of less massive haloes).

This thesis concludes that the observed properties of galaxy populations result from the combination of their natural characteristics and surrounding environments. Galaxy intrinsic luminosity (mass) determines the efficiency in forming stars, and extrinsic environmental influence accelerates the truncation of star formation.

Reproduced with permission. library@astro.utoronto.ca
July 16 2007