Do gas-poor galaxy clusters have different galaxy populations? The positive covariance of hot and cold baryons

TL;DR

This study investigates how gas-poor galaxy clusters with low X-ray surface brightness differ in galaxy populations, finding that richness correlates with gas content at fixed mass, but optical properties do not.

Contribution

It demonstrates that X-ray and optical properties are largely independent at fixed mass, except for richness, providing a new way to select low surface brightness clusters optically.

Findings

Gas-poor, low surface brightness clusters have 25% lower richness at fixed mass.

Richness correlates with gas content, but other optical properties do not show covariance with X-ray properties.

No anti-correlation between hot and cold baryons at fixed mass was confirmed.

Abstract

Galaxy clusters show a variety of intra-cluster medium properties at a fixed mass, among which gas fractions, X-ray luminosity and X-ray surface brightness. In this work we investigate whether the yet-undetermined cause producing clusters of X-ray low surface brightness also affects galaxy properties, namely richness, richness concentration, width and location of the red sequence, colour, luminosity, and dominance of the brightest cluster galaxy. We use SDSS-DR12 photometry and our analysis factors out the mass dependency to derive trends at fixed cluster mass. Clusters of low surface brightness for their mass have cluster richness in spite of their group-like luminosity. Gas-poor, low X-ray surface brightness, X-ray faint clusters for their mass, display 25\% lower richness for their mass at $4.4\sigma$ level. Therefore, richness and quantities depending on gas, such as gas fraction, $M_{gas}$, and X-ray surface brightness, are covariant at fixed halo mass. In particular, we do not confirm the hint of an anti-correlation of hot and cold baryons at fixed mass put forth in literature. All the remaining optical properties show no covariance at fixed mass, within the sensitivities allowed by our data and sample size. We conclude that X-ray and optical properties are disjoint, the optical properties not showing signatures of those processes involving gas content, apart from the richness-mass scaling relation. The covariance between X-ray surface brightness and richness is useful for an effective X-ray follow-up of low surface brightness clusters because it allows us to pre-select clusters using optical data of survey quality and prevent expensive X-ray observations.