LoCuSS: Pre-processing in galaxy groups falling into massive galaxy clusters at z=0.2

TL;DR

This study provides direct evidence that galaxy groups experience star formation quenching before falling into massive clusters, highlighting the importance of pre-processing in galaxy evolution.

Contribution

It presents the first robust observational evidence of pre-processing in galaxy groups infalling into clusters at z=0.2 using X-ray and spectroscopic data.

Findings

Lower star-forming fraction in infalling groups compared to cluster outskirts

Star formation quenching occurs prior to cluster entry

Groups are key sites for galaxy evolution processes

Abstract

We report direct evidence of pre-processing of the galaxies residing in galaxy groups falling into galaxy clusters drawn from the Local Cluster Substructure Survey (LoCuSS). 34 groups have been identified via their X-ray emission in the infall regions of 23 massive ($\rm \langle M_{200}\rangle = 10^{15}\,M_{\odot}$) clusters at $0.15<z<0.3$. Highly complete spectroscopic coverage combined with 24 $\rm\mu$m imaging from Spitzer allows us to make a consistent and robust selection of cluster and group members including star forming galaxies down to a stellar mass limit of $\rm M_{\star} = 2\times10^{10}\,M_{\odot}$. The fraction $\rm f_{SF}$ of star forming galaxies in infalling groups is lower and with a flatter trend with respect to clustercentric radius when compared to the rest of the cluster galaxy population. At $\rm R\approx1.3\,r_{200}$ the fraction of star forming galaxies in infalling groups is half that in the cluster galaxy population. This is direct evidence that star formation quenching is effective in galaxies already prior to them settling in the cluster potential, and that groups are favourable locations for this process.