Resurrecting the Red from the Dead: Optical Properties of BCGs in X-ray Luminous Clusters

TL;DR

This study analyzes the optical properties of brightest cluster galaxies (BCGs) in X-ray luminous clusters, revealing recent star formation in some BCGs and its implications for cluster evolution and optical studies.

Contribution

It provides new measurements of BCG surface brightness and colour profiles, highlighting the prevalence of recent star formation and its correlation with cluster X-ray properties.

Findings

25% of BCGs show blue cores indicating recent star formation

Blue-core BCGs are located near the cluster X-ray peak

BCGs with recent star formation are in clusters above the Lx-Tx relation

Abstract

We present measurements of surface brightness and colour profiles for the brightest cluster galaxies (BCGs) in a sample of 48 X-ray luminous galaxy clusters. These data were obtained as part of the Canadian Cluster Comparison Project (CCCP). The Kormendy relation of our BCGs is steeper than that of the local ellipticals, suggesting differences in the assembly history of these types of systems. We also find that while most BCGs show monotonic colour gradients consistent with a decrease in metallicity with radius, 25% of the BCGs show colour profiles that turn bluer towards the centre (blue-cores). We interpret this bluing trend as evidence for recent star formation. The excess blue light leads to a typical offset from the red sequence of 0.5 to 1.0 mag in (g'-r'), thus affecting optical cluster studies that may reject the BCG based on colour. All of the blue-core BCGs are located within ~10 kpc of the peak in the cluster X-ray emission. Furthermore, virtually all of the BCGs with recent star formation are in clusters that lie above the Lx-Tx relation. Based on photometry alone, these findings suggest that central star formation is a ubiquitous feature of BCGs in dynamically relaxed cool-core clusters. This implies that while AGNs and other heating mechanisms are effective at tempering cooling, they do not full compensate for the energy lost via radiation.