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. 
G. A. Ogrean, R. J. van Weeren, C. Jones, W. Forman, W. A. Dawson, N.   Golovich, F. Andrade-Santos, S. S. Murray, P. Nulsen...
astro-ph.CO astro-ph.GA astro-ph.HE