SuperModel Analysis of Abell 1246 and J255: on the Evolution of Galaxy Clusters from High to Low Entropy States

TL;DR

This paper uses entropy-based modeling of X-ray data to study galaxy clusters, revealing how their entropy profiles and nonthermal pressures evolve, and how mergers can temporarily reverse their entropy states.

Contribution

It introduces the SuperModel analysis of galaxy clusters' entropy profiles, highlighting the impact of mergers on their entropy evolution and the development of nonthermal pressure components.

Findings

Abell 1246 shows entropy flattening and ~20% nonthermal pressure support in outskirts.

J255's merger rejuvenated high entropy in outskirts, disrupting typical evolution.

Clusters evolve from high to low entropy states, but mergers can reverse this process.

Abstract

We present an analysis of high-quality X-ray data out to the virial radius for the two galaxy clusters Abell 1246 and GMBCG J255.34805+64.23661 (J255) by means of our entropy-based SuperModel. For Abell 1246 we find that the spherically-averaged entropy profile of the intracluster medium (ICM) progressively flattens outwards, and that a nonthermal pressure component amounting to ~20% of the total is required to support hydrostatic equilibrium in the outskirts; there we also estimate a modest value C~1.6 of the ICM clumping factor. These findings agree with previous analyses on other cool-core, relaxed clusters, and lend further support to the picture by Lapi et al. (2010) that relates the entropy flattening, the development of nonthermal pressure component, and the azimuthal variation of ICM properties to weakening boundary shocks. In this scenario clusters are born in a high-entropy state throughout, and are expected to develop on similar timescales a low entropy state both at the center due to cooling, and in the outskirts due to weakening shocks. However, the analysis of J255 testifies how such a typical evolutionary course can be interrupted or even reversed by merging especially at intermediate redshift, as predicted by Cavaliere et al. (2011b). In fact, a merger has rejuvenated the ICM of this cluster at z~0.45 by reestablishing a high entropy state in the outskirts, while leaving intact or erasing only partially the low-entropy, cool core at the center.