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

Abstract

Context. 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. Aims. 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. Methods. 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. Results. 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. Conclusions. 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.

Physical cool-core condensation radius in massive galaxy clusters

“…A comprehensive and systematic comparison of the spectroscopic mass deposition rate to the star formation rate in the BCG and the presence of molecular gas in the cluster core, significantly extending the small sample explored in Molendi et al (2016), would provide very effective constraints on the baryonic cycle in clusters. In addition, the detection of very diffuse, lowtemperature ICM in the center of non-cool-core clusters by the next generation of X-ray bolometers, may provide support for a scenario in which cool-core clusters rapidly switch into the non cool-core phase and vice versa (see Molendi et al 2023).…”

“…This is now considered a standard condition of the ICM in cool-core clusters, which represent ∼70% of the low-redshift population of X-ray flux-limited sample, according to Hudson et al (2010). On the other hand, some amount of cold and multiphase gas is now commonly observed in the submm and optical band in star forming regions of the BCG thanks to ALMA (e.g., McNamara et al 2014;Russell et al 2017;Temi et al 2018;Tremblay et al 2018;Rose et al 2019;North et al 2021) and MUSE (e.g., Olivares et al 2019Olivares et al , 2022Maccagni et al 2021), respectively. The presence of multiphase gas suggests that short-lived cooling flows raining all the way down onto the central supermassive black hole (SMBH) in the BCG have time to replenish the cold gas reservoir before being quenched by the ensuing SMBH feedback process.…”