Title: A2125 and its Environs: Evidence for an X-ray-emitting Hierarchical Superstructure

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 about 11 Mpc at redshift z = 0.247. The multi-peak X-ray morphology of A2125 suggests that the cluster is an ongoing coalescence of at least three major subunits. The dynamical youth of this cluster is consistent with its large fraction of blue galaxies observed by Butcher & Oemler. The superstructure contains two additional clusters, projected at distances of only 3 and 4.3 Mpc from A2125. But the most interesting feature is the low-surface-brightness X-ray emission from a moderate galaxy concentration not associated with individual clusters. The emission likely arises in a hot intergalactic medium, as predicted in N-body/hydro simulations of structure formation.