Velocity Dispersions of Brightest Cluster Galaxies and Their Host Clusters

TL;DR

This study investigates the relationship between the velocity dispersions of brightest cluster galaxies and their host clusters, revealing a tight correlation and a decreasing ratio with cluster mass, challenging some simulation predictions.

Contribution

It provides the first detailed analysis of the relation between BCG stellar velocity dispersion and cluster velocity dispersion using a large spectroscopic sample.

Findings

The relation between BCG velocity dispersion and cluster velocity dispersion is very tight.

The ratio of BCG to cluster velocity dispersion decreases with increasing cluster mass.

BCG formation appears more efficient in lower mass halos.

Abstract

We explore connections between brightest cluster galaxies (BCGs) and their host clusters. We first construct a HeCS-omnibus cluster sample including 227 galaxy clusters within $0.02 < z < 0.30$; the total number of spectroscopic members from MMT/Hectospec and SDSS observations is 52325. Taking advantage of the large spectroscopic sample, we compute physical properties of the clusters including the dynamical mass and cluster velocity dispersion ($\sigma_{cl}$). We also measure the central stellar velocity dispersion of the BCGs ($\sigma_{*,BCGs}$) to examine the relation between BCG velocity dispersion and cluster velocity dispersion for the first time. The observed relation between BCG velocity dispersion and the cluster velocity dispersion is remarkably tight. Interestingly, the $\sigma_{*, BCG} / \sigma_{cl}$ ratio decreases as a function of $\sigma_{cl}$ unlike the prediction from the numerical simulation of Dolag et al. (2010). The trend in $\sigma_{*, BCG} / \sigma_{cl}$ suggests that the BCG formation is more efficient in lower mass halos.