Does Concentration Drive the Scatter in the Stellar-to-Halo Mass Relation of Galaxy Clusters?

TL;DR

This study investigates how dark matter halo concentration influences the scatter in the stellar-to-halo mass relation of galaxy clusters, revealing a link between stellar mass, halo concentration, and large-scale bias.

Contribution

It provides the first robust observational evidence connecting halo concentration with stellar mass and assembly bias in galaxy clusters.

Findings

Low-mass star clusters have lower halo concentration than high-mass ones.

Clusters with lower stellar mass show higher large-scale bias.

Halo mass is similar across subsamples, but concentration varies significantly.

Abstract

Concentration is one of the key dark matter halo properties that could drive the scatter in the stellar-to-halo mass relation of massive clusters. We derive robust photometric stellar masses for a sample of brightest central galaxies (BCGs) in SDSS redMaPPer clusters at $0.17<z<0.3$, and split the clusters into two equal-halo mass subsamples by their BCG stellar mass $M_*$. The weak lensing profiles $\Delta\Sigma$ of the two cluster subsamples exhibit different slopes on scales below 1 M$pc/h$. To interpret such discrepancy, we perform a comprehensive Bayesian modelling of the two $\Delta\Sigma$ profiles by including different levels of miscentring effects between the two subsamples as informed by X-ray observations. We find that the two subsamples have the same average halo mass of $1.74 \times 10^{14} M_{\odot}/h$, but the concentration of the low-$M_*$ clusters is $5.87_{-0.60}^{+0.77}$, ${\sim}1.5\sigma$ smaller than that of their high-$M_*$ counterparts~($6.95_{-0.66}^{+0.78}$). Furthermore, both cluster weak lensing and cluster-galaxy cross-correlations indicate that the large-scale bias of the low-$M_*$, low-concentration clusters are ${\sim}10\%$ higher than that of the high-$M_*$, high-concentration systems, hence possible evidence of the cluster assembly bias effect. Our results reveal a remarkable physical connection between the stellar mass within 20{-}30 k$pc/h$, the dark matter mass within ${\sim}$ 200 k$pc/h$, and the cosmic overdensity on scales above 10 M$pc/h$, enabling a key observational test of theories of co-evolution between massive clusters and their central galaxies.