Richness Dependence of the Recent Evolution of Clusters of Galaxies

TL;DR

This study investigates the recent dynamical evolution of galaxy clusters, revealing that cluster richness influences their evolution, with poor clusters virializing and rich clusters possibly experiencing secondary infall, while X-ray luminosity remains unchanged.

Contribution

It provides new insights into how cluster richness affects their dynamical evolution and introduces a detailed analysis of ellipticity, velocity dispersion, and X-ray luminosity trends.

Findings

Poor clusters show increasing ellipticity with redshift, indicating virialization.

Rich clusters exhibit decreasing ellipticity, suggesting secondary infall.

Velocity dispersion increases over time, independent of cluster richness.

Abstract

We revisit the issue of the recent dynamical evolution of clusters of galaxies using a sample of ACO clusters with z<0.14, which has been selected such that it does not contain clusters with multiple velocity components nor strongly merging or interacting clusters, as revealed in X-rays. We use as proxies of the cluster dynamical state the projected cluster ellipticity, velocity dispersion and X-ray luminosity. We find indications for a recent dynamical evolution of this cluster population, which however strongly depends on the cluster richness. Poor clusters appear to be undergoing their primary phase of virialization, with their ellipticity increasing with redshift with a rate de/dz ~ 2.5, while the richest clusters show an ellipticity evolution in the opposite direction (with de/dz ~ -1.2), which could be due to secondary infall. When taking into account sampling effects due to the magnitude-limited nature of the ACO cluster catalogue we find no significant evolution of the cluster X-ray luminosity, while the velocity dispersion increases with decreasing redshift, independent of the cluster richness, at a rate dsigma/dz ~ -1700 km/sec.