Growth of brightest cluster galaxies via mergers since z = 1

TL;DR

This study examines the merger history of Brightest Cluster Galaxies (BCGs) since redshift 1, revealing significant variation in merging activity and suggesting substantial stellar mass growth over this period.

Contribution

It provides new observational evidence of the number and types of mergers BCGs experience at high redshift using HST data, highlighting differences in merging histories.

Findings

Average of 6.45 companion galaxies within 50 kpc of BCGs.

Equal split between major and minor mergers among companions.

Potential BCG mass increase by up to 1.8 times since z=1.

Abstract

Hierarchical assembly within clusters of galaxies is tied directly to the evolution of the Brightest Cluster Galaxies (BCGs), which dominate the stellar light in the centres of rich clusters. In this paper we investigate the number of mergers onto BCGs in 14 X-ray selected clusters over the redshift range 0.8 < z < 1.4 using HST imaging data. We find significant differences in the numbers of companion galaxies to BCGs between the clusters in our sample indicating that BCGs in similar mass clusters can have very different merging histories. Within a 50 kpc radius around the BCGs we find an average of 6.45 \pm 1.15 companion galaxies with mass ratios (companion:BCG) between 1:1 and 1:20. The infalling companions show a 50/50 split between major (1:1 - 1:2) and minor (1:3 - 1:20) mergers. When compared to similar work using lower redshift clusters, these results demonstrate that both major and minor merging was more common in the past. Since the dynamical timescales for merging onto the BCG are relatively short compared with the look-back time to z ~ 1 our results suggest that the BCG stellar mass may increase by as much as 1.8 times since z = 1. However the growth rate of BCGs will be substantially less if stripped material from nearby companions ends up in the diffuse intracluster light.