The accretion of galaxies into groups and clusters

We use the galaxy stellar mass and halo merger tree information from the semi-analytic model galaxy catalogue of Font et al. to examine the accretion of galaxies into a large sample of groups and clusters, covering a wide range in halo mass (1012.9 to 1015.3 h-1 M(circle dot)), and selected from each of four redshift epochs (z = 0, 0.5, 1.0 and 1.5). We find that clusters at all examined redshifts have accreted a significant fraction of their final galaxy populations through galaxy groups. A 1014.5 h-1 M(circle dot) mass cluster at z = 0 has, on average, accreted similar to 40 per cent of its galaxies (M(stellar) > 109 h-1 M(circle dot)) from haloes with masses greater than 1013 h-1 M(circle dot). Further, the galaxies which are accreted through groups are more massive, on average, than the galaxies accreted through smaller haloes or from the field population. We find that at a given epoch, the fraction of galaxies accreted from isolated environments is independent of the final cluster or group mass. In contrast, we find that observing a cluster of the same halo mass at each redshift epoch implies different accretion rates of isolated galaxies, from 5 to 6 per cent per Gyr at z = 0 to 15 per cent per Gyr at z = 1.5. We find that combining the existence of a Butcher-Oemler effect at z = 0.5 and the observations that galaxies within groups display significant environmental effects with galaxy accretion histories justifies striking conclusions. Namely that the dominant environmental process must begin to occur in haloes of 1012-1013 h-1 M(circle dot), and act over time-scales of > 2 Gyr. This argues in favour of a mechanism like 'strangulation', in which the hot halo of a galaxy is stripped upon infalling into a more massive halo. This simple model predicts that by z = 1.5 galaxy groups and clusters will display little to no environmental effects. This conclusion may limit the effectiveness of red sequence cluster-finding methods at high redshift.