Tracing the assembly histories of galaxy clusters in the nearby universe

TL;DR

This study analyzes 67 nearby galaxy clusters to understand their assembly history, revealing prevalent substructures, diverse central galaxy roles, and complex co-evolution patterns between galaxies and intracluster media.

Contribution

It provides a new classification of cluster assembly states based on substructure significance and highlights the complex co-evolution of central galaxies and their clusters.

Findings

70% of clusters contain substructures

19% of clusters have non-central brightest galaxies

No correlation between CDG offset and peculiar velocity

Abstract

We have compiled a sample of 67 nearby ($z$ < 0.15) clusters of galaxies, for which on average more than 150 spectroscopic members are available, and, by applying different methods to detect substructures in their galaxy distribution, we have studied their assembly history. Our analysis confirms that substructures are present in 70% of our sample, having a significant dynamical impact in 57% of them. A classification of the assembly state of the clusters based on the dynamical significance of their substructures is proposed. In 19% of our clusters, the originally identified brightest cluster galaxy is not the central gravitationally dominant galaxy (CDG), but turns out to be either the second-rank, or the dominant galaxy of a substructure (a SDG, in our classification), or even a possible "fossil" galaxy in the periphery of the cluster. Moreover, no correlation was found in general between the projected offset of the CDG from the X-ray peak and its peculiar velocity. The comparison of the CDGs properties with the assembly states and dynamical state of the intracluster media, especially the core cooling status, suggests a complex assembly history, with clear evidence of co-evolution of the CDG and its host cluster in the innermost regions.