How special are Brightest Group and Cluster Galaxies?

TL;DR

This study analyzes the unique properties of Brightest Group and Cluster Galaxies (BCGs) using SDSS data, revealing their distinct size, dynamics, stellar populations, and active galactic nucleus activity compared to similar non-BCGs.

Contribution

It provides new insights into the structural, dynamical, and stellar population differences of BCGs, including improved algorithms for identifying BCGs and correcting luminosity measurements.

Findings

BCGs are larger and have higher velocity dispersions than non-BCGs of the same stellar mass.

BCGs follow a different fundamental plane and have a steeper Faber-Jackson relation.

BCGs are more likely to host radio-loud AGN and have higher alpha/Fe ratios.

Abstract

We use the Sloan Digital Sky Survey to construct a sample of 625 brightest group and cluster galaxies (BCGs) together with control samples of non-BCGs matched in stellar mass, redshift, and color. We investigate how the systematic properties of BCGs depend on stellar mass and on their privileged location near the cluster center. The groups and clusters that we study are drawn from the C4 catalogue of Miller et al. (2005) but we have developed improved algorithms for identifying the BCG and for measuring the cluster velocity dispersion. Since the SDSS photometric pipeline tends to underestimate the luminosities of large galaxies in dense environments, we have developed a correction for this effect which can be readily applied to the published catalog data. We find that BCGs are larger and have higher velocity dispersions than non-BCGs of the same stellar mass, which implies that BCGs contain a larger fraction of dark matter. In contrast to non-BCGs, the dynamical mass-to-light ratio of BCGs does not vary as a function of galaxy luminosity. Hence BCGs lie on a different fundamental plane than ordinary elliptical galaxies. BCGs also follow a steeper Faber-Jackson relation than non-BCGs, as suggested by models in which BCGs assemble via dissipationless mergers along preferentially radial orbits. We find tentative evidence that this steepening is stronger in more massive clusters. BCGs have similar mean stellar ages and metallicities to non-BCGs of the same mass, but they have somewhat higher alpha/Fe ratios, indicating that star formation may have occurred over a shorter timescale in the BCGs. Finally, we find that BCGs are more likely to host radio-loud active galactic nuclei than other galaxies of the same mass, but are less likely to host an optical AGN. The differences we find are more pronounced for the less massive BCGs.