Complex X-ray morphology of Abell 3128 : a distant cluster behind a disturbed cluster

Abstract

We present here the results of a detailed study of the X-ray properties of the cluster of galaxies Abell 3128 (z = 0.06), based on the analysis of deep (100 ks) XMM-Newton data. The most obvious feature of the X-ray morphology of A3128 is the presence of two X-ray peaks separated by ∼12'. By detecting the redshifted Fe K line, we find that the Northeast (NE) X-ray peak observed toward A3128 is a distant luminous cluster of galaxies at redshift z = 0.44. Our subsequent optical spectroscopic observation of a distant radio bright galaxy in the centre of the NE X-ray peak with the Magellan telescope also revealed a redshift of z = 0.44, confirming the association of the galaxy with the cluster seen in X-rays. We detect a gravitational arc around the galaxy. The properties of this galaxy indicate that it is the cD galaxy of the cluster in the background. The properties of the Southwest X-ray peak suggest that it is the core of a group merging with A3128 along our line of sight. Based on 2D maps of thermodynamic properties of the intra-cluster medium determined after subtracting a model for the background cluster, we conclude that an enhanced surface brightness region at a distance of ∼2.8' from the centre of the galaxy distribution is the centre of the gravitational potential of the cluster A3128. The unrelaxed nature of A3128 can be attributed to its location in the high density environment of the Horologium-Reticulum supercluster.