The Impact of Environment on the Stellar Mass - Halo Mass Relation

TL;DR

This paper demonstrates that the magnitude gap between the brightest galaxies in a cluster significantly influences the stellar mass-halo mass relation, reducing scatter and linking BCG growth to halo assembly.

Contribution

It introduces the magnitude gap as a key factor in the SMHM relation, quantifies its impact, and connects BCG growth to hierarchical mergers in galaxy clusters.

Findings

Magnitude gap explains a significant variance in the SMHM relation.

Including the gap reduces the intrinsic scatter in BCG stellar mass.

The relationship is supported by both observations and simulations.

Abstract

A large variance exists in the amplitude of the Stellar Mass - Halo Mass (SMHM) relation for group and cluster-size halos. Using a sample of 254 clusters, we show that the magnitude gap between the brightest central galaxy (BCG) and its second or fourth brightest neighbor accounts for a significant portion of this variance. We find that at fixed halo mass, galaxy clusters with a higher magnitude gap have a higher BCG stellar mass. This relationship is also observed in semi-analytic representations of low-redshift galaxy clusters in simulations. This SMHM-magnitude gap stratification likely results from BCG growth via hierarchical mergers and may link assembly of the halo with the growth of the BCG. Using a Bayesian model, we quantify the importance of the magnitude gap in the SMHM relation using a multiplicative stretch factor, which we find to be significantly non-zero. The inclusion of the magnitude gap in the SMHM relation results in a large reduction in the inferred intrinsic scatter in the BCG stellar mass at fixed halo mass. We discuss the ramifications of this result in the context of galaxy formation models of centrals in group and cluster-sized halos.