Cool core remnants in galaxy clusters

Context. X ray clusters are conventionally divided into two classes: “cool core” (CC) and “non-cool core” (NCC) objects, on the basis of the observational properties of their central regions. Recent results have shown that the cluster population is bimodal.
Aims: We want to understand whether the observed distribution of clusters is due to a primordial division into two distinct classes or rather to differences in the way these systems evolve across cosmic time.
Methods: We systematically search the ICM of NCC clusters in a subsample of the B55 flux limited sample of clusters for regions which have some characteristics typical of cool cores, namely low entropy gas and high metal abundance.
Results: We find that most NCC clusters in our sample host regions reminiscent of CC, i.e. characterized by relative low entropy gas (albeit not as low as in CC systems) and a metal abundance excess. We have dubbed these structures “cool core remnants”, since we interpret them as the remains of a cool core after a heating event (AGN giant outbursts in a few cases and more commonly mergers). We infer that most NCC clusters have undergone a cool core phase during their life. The fact that most cool core remnants are found in dynamically active objects provides strong support to scenarios where cluster core properties are not fixed “ab initio” but evolve across cosmic time.

Appendices are only available in electronic form at http://www.aanda.org