Frontier Fields Clusters: Deep Chandra Observations of the Complex
  Merger MACS J1149.6+2223
  The HST Frontier Fields cluster MACS J1149.6+2223 is one of the most complex
merging clusters, believed to consist of four dark matter halos. We present
results from deep (365 ks) Chandra observations of the cluster, which reveal
the most distant cold front (z=0.544) discovered to date. In the cluster
outskirts, we also detect hints of a surface brightness edge that could be the
bow shock preceding the cold front. The substructure analysis of the cluster
identified several components with large relative radial velocities, thus
indicating that at least some collisions occur almost along the line of sight.
The inclination of the mergers with respect to the plane of the sky poses
significant observational challenges at X-ray wavelengths. MACS J1149.6+2223
possibly hosts a steep-spectrum radio halo. If the steepness of the radio halo
is confirmed, then the radio spectrum, combined with the relatively regular ICM
morphology, could indicate that MACS J1149.6+2223 is an old merging cluster.
