The Off‐Axis Galaxy Cluster Merger A115

Abstract

We have used a 50.4 ks Chandra observation to examine the galaxy cluster A115 (z = 0.1971). The X-ray surface brightness distribution shows a merger of two subclusters with substantial off-axis motion. The northern subcluster moves toward the southwest and exhibits a strong X-ray surface brightness gradient toward this direction as well as a fan of X-ray-bright intracluster medium (ICM) toward the northeast. The southern subcluster shows a pronounced northeast-southwest elongation, probably as a consequence of the ongoing merger. The temperature of the ICM is highly nonuniform across the cluster. Although the merger is well underway and has had a substantial influence on the morphology of the two subclusters, it has apparently not disturbed the cool subcluster cores; the cores show projected temperatures of ~3 keV and are substantially cooler than the surrounding plasma of temperature ~5 keV. The ICM in between the subclusters has a significantly elevated temperature of ~8 keV. However, owing to the low surface brightness, we do not find direct evidence for shocks in this region. The X-ray surface brightness distribution of the northern subcluster is highly nonuniform. The data show a bright central core region of about 10'' (30 kpc) diameter. A highly significant surface brightness discontinuity separates the core from the ambient ICM. We discuss several possibilities to explain the compact core and its relation to the central radio galaxy 3C 28.0 and the subcluster's cooling flow. Farther outward, at about 246 (80 kpc) southwest of the subcluster core, the surface brightness distribution shows a jump associated with a contact discontinuity. The Chandra imaging spectroscopy data do not show evidence for a shock associated with the surface brightness jump. We conclude that it is associated with denser plasma moving subsonically through the ambient plasma.