ChandraObservation of A2256: A Cluster at the Early Stage of Merging

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Abstract
We present Chandra observations of the rich cluster of galaxies A2256. In addition to the known cool subcluster, a new structure (we call it the "shoulder" in this paper, based on its morphology) was resolved 2' east of the peak of the main cluster. It is shown as a localized feature embedded in the main cluster. Its position is roughly at the center of a low-brightness radio relic. Spectral analysis shows that the shoulder has a high iron abundance, ~1 (after the decomposition). The gas mass within it is around 2 × 1011 M☉. We suggest that this structure is either another merging component or an internal structure of the main cluster. The previously known subcluster has a low temperature (~4.5 keV) and high iron abundance (~0.6) in the central 150 kpc. The main cluster has a temperature of 7-8 keV and an iron abundance of 0.2-0.3 around the center. The Chandra image shows a relatively sharp brightness gradient at the south of the subcluster peak running south-southeast. A temperature jump was found across the edge, with lower temperature inside the subcluster. This phenomenon is qualitatively similar to the "cold fronts" found in A2142 and A3667. While a simple interpretation is not possible because of the projection, the edge indicates relative motion and contact of the two gas clouds. The Chandra temperature map shows only moderate temperature variations across the cluster, but not as strong as those expected in a major merger. If the shoulder is ignored, the temperature map resembles simulations at the early stage of merging while the subcluster approached the main cluster from somewhere west. The observed temperature map and the edgelike feature near the south of the subcluster imply that the ongoing merger is still at the early stage. The X-ray redshifts of several regions were measured. The results are consistent with a single value and all agree with the optical value. At least three member galaxies, including a radio head-tail galaxy, were found to have corresponding X-ray emission, with X-ray luminosity from several times 1040 to 1041 ergs s-1. We find that the observed characteristics (temperature and iron abundance gradient) of the subcluster are similar to those of some poor clusters. The absence of galaxies around the peak of the subcluster is proposed to be the result of different falling velocities of the galaxies and the core gas.