This X-ray image shows the cluster RXCJ0658.5-5556, a violently merging cluster. The dark blue region near the centre is the core of a sub-cluster flying through the main cluster body at high ‘supersonic’ speed, as indicated by a mach cone in front of this region. This sub-cluster had a pronounced low entropy core region that so far survived and is still visible.
In front of the bow shock region (to the right of the blue region) we see a very dramatic entropy enhancement marked by an arrow which is the cluster gas heated by the shock front caused by the fast flying sub-cluster ‘bullet’. This is the first direct X-ray evidence of shock heating in merging clusters.