Vol. 570
In section 3. Cosmology

The stripping of a galaxy group diving into the massive cluster A2142

by D. Eckert, S. Molendi, M. Owers, et al. A&A 570, A119


Large structures such as galaxy clusters form hierarchically in the universe by merging the subentities. Even though major mergers of clusters, like the bullet, are easy to see and study, and individual galaxies have also been frequently observed to fall inward and be stripped of gas in local clusters, such as Virgo or Coma, an infalling group is not very frequent. This work reports the discovery of a diffuse, irregular X-ray feature in the outskirts of the massive cluster Abell 2142, which is identified as an infalling galaxy group. The feature has a core-tail structure, where the core closer to the cluster center coincides with a group of galaxies. The tail pointing away from the cluster is 800kpc long, the longest X-ray tail ever reported. The gas of the tail is not as hot as the ambient X-ray gas, confirming it comes from a smaller structure. This is gas stripped from the infalling group. The 3D collisional velocity is approximately 1200 km/s. The survival of the tail in the hot X-ray medium of the cluster is 400 times longer than the conduction time scale. Conduction must be suppressed through a tangled magnetic field with a short coherence length and with plasma microinstabilities. With this long survival time, it is possible that the infalling material can eventually settle within the core of the main cluster.