The evolving cluster cores: Putting together the pieces of the puzzle

TL;DR

This paper investigates whether cool core and non-cool core galaxy clusters are distinct or represent different evolutionary states, analyzing spectral data and predicting future observational capabilities to detect residual cool gas.

Contribution

It introduces a model showing CCs and NCCs are dynamic states of clusters, supported by spectral analysis and simulations predicting future detection of residual cool gas.

Findings

Residual cool gas in NCCs can have short cooling times similar to CCs.

Future XRISM and ATHENA spectrometers can detect low ionization Fe lines from cool gas.

Clusters transition between CC and NCC states through mergers and reformation processes.

Abstract

In this work we address the issue of whether the division of clusters in cool cores (CCs) and non-cool cores (NCCs) is due to a primordial difference or to how clusters evolve across cosmic time. Our first goal is to establish if spectra from the central regions of a subclass of NCCs known as cool core remnants (CCRs) are consistent with having a small but significant amount of short cooling time gas, thereby allowing a transformation to CC systems on a timescale of a giga year. Our second goal is to determine if low ionization Fe lines emitted from this residual cool gas will be detectable by the calorimeters that will fly on board XRISM and ATHENA. We performed a spectral analysis of CCR systems with a multi temperature model and, assuming the different components to be in pressure equilibrium with one another, derived entropy and cooling time distributions for the X-ray emitting gas. We find that in most of our systems, the spectral model allows for a fraction of low entropy, short cooling time gas with a mass that is comparable to the one in CC systems. Moreover, simulations show that future spectrometers on board XRISM and ATHENA will have the power to directly resolve emission lines from the low temperature gas, thereby providing incontrovertible evidence for its presence. Within the scenario that we have explored, the constant fraction of CCs measured across cosmic time emerges from a dynamical equilibrium where CCs transformed in NCCs through mergers are balanced by NCCs that revert to CCs. Furthermore, CCs and NCCs should not be viewed as distinct sub classes, but as ``states" between which clusters can move.