EVOLUTION OF GROUP GALAXIES FROM THE FIRST RED-SEQUENCE CLUSTER SURVEY

We study the evolution of the red-galaxy fraction (fred) in 905 galaxy groups with 0.15 ≤ z < 0.52. The galaxy groups are identified by the 'probability friends-of-friends' algorithm from the first Red-Sequence Cluster Survey (RCS1) photometric-redshift sample. There is a high degree of uniformity in the properties of the red sequence of the group galaxies, indicating that the luminous red-sequence galaxies in the groups are already in place by z ∼ 0.5 and that they have a formation epoch of z ∼> 2. In general, groups at lower redshifts exhibit larger fred than those at higher redshifts, showing a group Butcher-Oemler effect. We investigate the evolution of fred by examining its dependence on four parameters, one of which can be classified as intrinsic and three of which can be classified as environmental: galaxy stellar mass (M*), total group stellar mass (M*,grp, a proxy for group halo mass), normalized group-centric radius (rgrp), and local galaxy density (Σ5). We find that M* is the dominant parameter such that there is a strong correlation between fred and galaxy stellar mass. Furthermore, the dependence of fred on the environmental parameters is also a strong function of M*. Massive galaxies (M* ∼> 1011 M☉) show little dependence of fred on rgrp, M*,grp, and Σ5 over the redshift range. The dependence of fred on these parameters is primarily seen for galaxies with lower masses, especially for M* ∼< 1010.6 M☉. We observe an apparent 'group down-sizing' effect, in that galaxies in lower-mass halos, after controlling for galaxy stellar mass, have lower fred. We find a dependence of fred on both rgrp and Σ5 after the other parameters are controlled. At a fixed rgrp, there is a significant dependence of fred on Σ5, while rgrp gradients of fred are seen for galaxies in similar Σ5 regions. This indicates that galaxy group environment has a residual effect over that of local galaxy density (or vice versa), and both parameters need to be considered. This result suggests that processes identified with local galaxy density, such as galaxy harassment and mergers, and those associated with accretion into a larger group halo, such as ram pressure and strangulation, are both partaking in driving galaxies to their final red quiescent state. We discuss these results in the context of the 'nature versus nurture' scenario of galaxy evolution.