Pre-Processing and Post-Processing in Group-Cluster Mergers
  Galaxies in clusters are more likely to be of early type and to have lower
star formation rates than galaxies in the field. Recent observations and
simulations suggest that cluster galaxies may be `pre-processed' by group or
filament environments and that galaxies that fall into a cluster as part of a
larger group can stay coherent within the cluster for up to one orbital period
(`post-processing'). We investigate these ideas by means of a cosmological
$N$-body simulation and idealized $N$-body plus hydrodynamics simulations of a
group-cluster merger. We find that group environments can contribute
significantly to galaxy pre-processing by means of enhanced galaxy-galaxy
merger rates, removal of galaxies' hot halo gas by ram pressure stripping, and
tidal truncation of their galaxies. Tidal distortion of the group during infall
does not contribute to pre-processing. Post-processing is also shown to be
effective: galaxy-galaxy collisions are enhanced during a group's pericentric
passage within a cluster, the merger shock enhances the ram pressure on group
and cluster galaxies, and an increase in local density during the merger leads
to greater galactic tidal truncation.
