Cool core remnants in galaxy clusters

TL;DR

This study reveals that many non-cool core galaxy clusters contain remnants of cool cores, suggesting that cluster core properties evolve over cosmic time rather than being fixed from formation.

Contribution

It introduces the concept of cool core remnants in NCC clusters and provides evidence for the evolutionary link between CC and NCC galaxy clusters.

Findings

Most NCC clusters host cool core remnants with low entropy and high metal abundance.

Cool core remnants are more common in dynamically active clusters.

Results support the idea that cluster cores evolve over cosmic time.

Abstract

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 (Cavagnolo et al. 2009). We want to understand whether the observed distribution of clusters is due to a primordial division into two distinct classes rather than to differences in how these systems evolve across cosmic time. 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 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 what 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.