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