Close companions to Brightest Cluster Galaxies: Support for minor mergers and downsizing

TL;DR

This study investigates the close companions of Brightest Cluster Galaxies to quantify their merger-driven growth, finding that BCGs have more companions and likely grow significantly through minor mergers, especially at higher redshifts.

Contribution

It provides new observational evidence for the role of minor mergers in BCG growth and quantifies the number and luminosity of companions down to 20:1 ratios using deep photometric data.

Findings

BCGs have more close companions than similar luminous galaxies.

A significant fraction of identified companions are real physical neighbors.

Minor mergers could contribute up to 10% mass increase over 0.5 Gyr at z~0.3.

Abstract

We identify close companions of Brightest Cluster Galaxies (BCGs) for the purpose of quantifying the rate at which these galaxies grow via mergers. By exploiting deep photometric data from the CFHTLS, we probe the number of companions per BCG (Nc) with luminosity ratios down to those corresponding to potential minor mergers of 20:1. We also measure the average luminosity in companions per galaxy (Lc). We find that Nc and Lc rise steeply with luminosity ratio for both the BCGs, and a control sample of other bright, red, cluster galaxies. The trend for BCGs rises more steeply, resulting in a larger number of close companions. For companions within 50kpc of a BCG, Nc= 1.38+/-0.14 and Lc=(2.14+/-0.31)x10^(10)L_sun and for companions within 50kpc of a luminosity matched control sample of non-BCGs, Nc=0.87+/-0.08 and Lc=(1.48+/-0.20)x10^(10)L_sun. This suggests that the BCGs are likely to undergo more mergers compared to otherwise comparable luminous galaxies. Additionally, compared to a local sample of luminous red galaxies, the more distant sample presented in this study (with redshifts between 0.15-0.39,) shows a higher Nc, suggesting the younger and smaller BCGs are still undergoing hierarchical formation. Using the Millennium Simulations we model and estimate the level of contamination due to unrelated cluster galaxies. The contamination by interloping galaxies is 50% within projected separations of 50kpc, but within 30kpc, 60% of identified companions are real physical companions. We conclude that the luminosity of bound merger candidates down to luminosity ratios of 20:1 could be adding as much as 10% to the mass of a typical BCG over 0.5Gyr at redshifts of z~0.3.