Pre-Processing and Post-Processing in Group-Cluster Mergers

TL;DR

This paper investigates how galaxy groups influence galaxy evolution before and after cluster infall, using simulations to identify mechanisms like mergers, ram pressure stripping, and tidal effects that contribute to galaxy transformation.

Contribution

It provides a detailed simulation-based analysis of pre- and post-processing effects in galaxy group-cluster mergers, highlighting specific physical mechanisms involved.

Findings

Group environments increase galaxy merger rates.

Ram pressure stripping removes hot halo gas from galaxies.

Tidal truncation affects galaxy sizes during infall.

Abstract

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.