The X-ray subcluster in A 2256: cluster mergers, cooling flows and diffuse radio sources

Abstract

ROSAT X-ray images of the cluster A2256 have recently resolved the cluster core into two peaks, separated by a projected distance of about 3.5 arcmin. The second peak is interpreted as a lower temperature subcluster in the process of merging. Motivated by this observation, we discuss the fate of the gas in a merging subcluster and show that it may not be shocked during the merger. In particular, if either cluster or subcluster contains a cooling flow the cooling gas will not necessarily be reheated but can thermalize its bulk kinetic energy in the less dense hot gas. As clusters evolve via hierarchical mergers, cooling flows are interrupted as the intracluster medium is stirred up but are then quickly re-established. The main cluster in A2256 has no cooling flow, but we show that the subcluster itself did have. A merger of this type deposits large quantities of cold gas in the core of the main cluster and may promote a cooling flow there. The X-ray subcluster in A2256 coincides with a diffuse radio source and is close to several head-tail radio sources. We suggest that these sources were originally members of the subcluster and that ram-pressure stripping of the surrounding subcluster core has created the diffuse radio source from their lobes.