Substructures in galaxy clusters: a comparative X-ray and photometric analysis of the REXCESS sample

TL;DR

This study compares the substructure content of galaxy clusters using X-ray and optical data, revealing that multiple indicators are needed for accurate morphological classification and that dynamical state does not always predict future mass assembly.

Contribution

It introduces a multi-indicator approach to assess galaxy cluster substructures and compares optical and X-ray observations to understand cluster dynamics better.

Findings

Approximately 35% of clusters are disturbed based on key indicators.

Main indicators poorly correlate with secondary measures, indicating complex substructure dynamics.

Dynamical state does not necessarily predict future mass accretion.

Abstract

The main goals of the present work are (i) to study the substructure content of a representative, X-ray selected sample of 31 galaxy clusters, as traced by the spatial distribution of their red-sequence galaxies, and (ii) to compare it to that observed in the intracluster medium distribution. Our substructure indicators are the asymmetry test, $\beta$, the residuals of the galaxy surface density map, $\Delta_\Sigma$, and the Fourier elongation, $FE$. We probe the clusters core with secondary tests: the offset between the central brightest cluster galaxy and the X-ray emission peak, $\Delta r_{\mathrm{BCG-X}}$, the magnitude offset between the first and second brightest galaxies, $\Delta m_{12}$, and their radial offset, $\Delta r_{12}$. The main indicators exhibit continuous distributions, making a discrete classification difficult. A partition based on $\beta$ and $\Delta_\Sigma$ gives a fraction $\sim35\%$ of disturbed systems; $\sim65\%$ of the clusters are disturbed according to at least one of these two quantities. The main indicators poorly correlate with the secondary quantities, likely due to substructures observed prior core crossing. Nine of the 12 X-ray disturbed clusters are also flagged as such by either $\beta$ or $\Delta_\Sigma$. Cool cores are hosted by systems with higher optical concentrations, smaller $\Delta r_{\mathrm{BCG-X}}$, and larger $\Delta r_{12}$; however, both populations do not differ significantly in terms of their overall morphology probed by the main indicators. Our results show the necessity of using a variety of independent tests and data sets to obtain a clear picture of a cluster's morphology. Furthermore, we find that a cluster's dynamical state, consequence of its recent merger history, is not necessarily representative of a future mass assembly via accretion of substructures.