Abstract

Based on a deep ROSAT/PSPC observation, we reveal an elongated complex of extended X-ray-emitting objects in and around the galaxy cluster A2125. Multicolor optical imaging of galaxies in the field suggests that this complex represents a hierarchical superstructure spanning ~11 h−150 Mpc at the redshift ~0.247. The multiple peak X-ray morphology and large blue galaxy fraction of A2125 indicate that the cluster is undergoing a coalescence of subunits. The superstructure contains two additional clusters, projected at distances of only 3 and 4.3 h−150 Mpc from A2125. The most interesting feature is, however, the low surface brightness X-ray emission from a moderate galaxy concentration away from individual clusters. The emission likely arises in a hot (~107 K) intergalactic medium, as predicted in N-body/hydro simulations of structure formation.

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