2011A&A...532A.123R

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 that can start a large-scale synchrotron emission (most likely mergers) are the same as those that can destroy CC, which 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 ≃3Gyr. This suggests that NCC transform into CC more rapidly than predicted from the cooling time, which is about 10Gyr in NCC systems, allowing the possibility of a cyclical evolution between the CC and NCC states.