Back and forth from cool core to non-cool core: clues from radio-halos

TL;DR

This study supports the evolutionary model of galaxy cluster core states, showing that radio halos are associated with non-cool core clusters and suggesting a cyclical transition between core states over less than 3 billion years.

Contribution

It provides observational evidence linking radio halos to non-cool core clusters, supporting the idea that mergers transform cluster cores and that these states can cycle over time.

Findings

Radio halos are only found in non-cool core clusters.

NCC clusters can relax into CC states in less than 3 Gyr.

The results favor evolutionary models over primordial models.

Abstract

X-ray astronomers often divide galaxy clusters into two classes: "cool core" (CC) and "non-cool core" (NCC) objects. The origin of this dichotomy has been the subject of debate in recent years, between "evolutionary" models (where clusters can evolve from CC to NCC, mainly through mergers) and "primordial" models (where the state of the cluster is fixed "ab initio" by early mergers or pre-heating). We found that in a well-defined sample (clusters in the GMRT Radio halo survey with available Chandra or XMM-Newton data), none of the objects hosting a giant radio halo can be classified as a cool core. This result suggests that the main mechanisms which can start a large scale synchrotron emission (most likely mergers) are the same that can destroy CC and therefore strongly supports "evolutionary" models of the CC-NCC dichotomy. Moreover combining the number of objects in the CC and NCC state with the number of objects with and without a radio-halo, we estimated that the time scale over which a NCC cluster relaxes to the CC state, should be larger than the typical life-time of radio-halos and likely shorter than about 3 Gyr. This suggests that NCC transform into CC more rapidly than predicted from the cooling time, which is about 10 Gyr in NCC systems, allowing the possibility of a cyclical evolution between the CC and NCC states.