On the dynamical state of galaxy clusters: insights from cosmological simulations II

TL;DR

This study compares dark matter and hydrodynamical simulations of galaxy clusters, revealing that the velocity dispersion deviation parameter can effectively indicate the clusters' dynamical state, offering a practical observational tool.

Contribution

It introduces the velocity dispersion deviation parameter as a new, observable proxy for the virial ratio to assess galaxy cluster dynamical states.

Findings

Hydrodynamical runs show ~10% lower virial ratio than dark matter runs.

No clear separation between relaxed and unrelaxed clusters using traditional parameters.

Linear correlation between virial ratio and velocity dispersion deviation parameter.

Abstract

Using a suite of cosmology simulations of a sample of $> 120$ galaxy clusters with $\log(M_{DM, vir}) \le 14.5$. We compare clusters that form in purely dark matter run and their counterparts in hydro runs and investigate 4 independent parameters, that are normally used to classify dynamical state. We find that the virial ratio $\eta$ in hydro-dynamical runs is $\sim 10$ per cent lower than in the DM run, and there is no clear separation between the relaxed and unrelaxed clusters for any parameter. Further, using the velocity dispersion deviation parameter $\zeta$, which is defined as the ratio between cluster velocity dispersion $\sigma$ and the theoretical prediction $\sigma_t = \sqrt{G M_{total}/R}$, we find that there is a linear correlation between the virial ratio $\eta$ and this $\zeta$ parameter. We propose to use this $\zeta$ parameter, which can be easily derived from observed galaxy clusters, as a substitute of the $\eta$ parameter to quantify the cluster dynamical state.