  We have used a 50.4 ksec Chandra obsffervation to examine the galaxy cluster
Abell 115 (z=0.1971). The X-ray surface brightness distribution shows a merger
of 2 sub-clusters with substantial off-axis motion. The northern sub-cluster
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 Intra-Cluster
Medium (ICM) toward the northeast. The southern sub-cluster shows a pronounced
northeast-southwest elongation, probably as a consequence of the ongoing
merger. The temperature of the ICM is highly non-uniform across the cluster.
Although the merger is well underway and has had a substantial influence on the
morphology of the 2 sub-clusters, it has apparently not disturbed the cool
sub-cluster cores. The ICM in between the sub-clusters 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 sub-cluster is highly non-uniform. 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 sub-cluster's cooling
flow. Further outwards, at about 24.6" (80 kpc) to the southwest of the
sub-cluster 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 sub-sonically through the
ambient plasma.
