Backsplash galaxies in isolated clusters

Abstract

At modest radii from the centre of galaxy clusters, individual galaxies may be infalling to the cluster for the first time, or have already visited the cluster core and are coming back out again. This latter population of galaxies is known as the backsplash population. Differentiating them from the infalling population presents an interesting challenge for observational studies of galaxy evolution. To attempt to do this, we assemble a sample of 14 redshift-isolated and spatially isolated galaxy clusters from the Sloan Digital Sky Survey. We clean this sample of cluster-cluster mergers to ensure that the galaxies contained within them are (to an approximation) only backsplashing from the centre of their parent clusters and are not being processed in subclumps. By stacking them together to form a composite cluster, we find evidence for both categories of galaxies at intermediate radii from the cluster centre. Application of mixture modelling to this sample then serves to differentiate the infalling galaxies (which we model on galaxies from the cluster outskirts) from the backsplash ones (which we model on galaxies in the high-density core with low-velocity offsets from the cluster mean). We find that the fraction of galaxies with populations similar to the low-velocity cluster core galaxies is f=-0.052R/Rvirial+ 0.612 ± 0.06, which we interpret as being the backsplash population fraction at 1 < 2. Although some interlopers may be affecting our results, the results are demonstrated to be in concordance with earlier studies in this area that support density-related mechanisms as being the prime factor in determining the star formation rate of a galaxy. © 2010 The Author Monthly Notices of the Royal Astronomical Society © 2010 RAS.