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

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 theirX-ray emission in the infall regions of 23 massive (< M-200 > = 10(15) M-circle dot) clusters at 0.15 < z < 0.3. Highly complete spectroscopic coverage combined with 24 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 M* = 2 x 10(10) M-circle dot. The fraction 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 R approximate to 1.3 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.