A Chandra view of the multiple merger in Abell 2744

We present a Chandra observation of the merging cluster of galaxies Abell 2744. The cluster shows strong evidence for an ongoing major merger which we believe to be responsible for the radio halo. X-ray emission and temperature maps of the cluster, combined with the spatial and redshift distribution of the galaxies, indicate a roughly north–south axis for the merger, with a significant velocity component along the line of sight. The merger is occurring at a very large velocity, with –3. We estimate the mass ratio of the merging subclusters to be of the order of unity. They are nearing their closest approach to each other, but with a significant impact parameter. In addition, there is a small merging subcluster toward the north-west, unrelated to the major merger, which shows evidence of a bow shock. A hydrodynamical analysis of the subcluster indicates a merger velocity corresponding to a Mach number of ∼1.2, consistent with a simple infall model. This infalling subcluster may also be re-exciting electrons in the radio halo. Its small Mach number lends support to turbulent reacceleration models for radio halo formation.