Gravity-Selected Galaxy Clusters: a Tight Mass-Richness relation and an unclear Compton $Y$-richness trend

TL;DR

This study demonstrates a very tight correlation between optical richness and weak-lensing mass in gravity-selected galaxy clusters, while the relation between Compton Y and richness remains unclear, highlighting the effectiveness of richness as a mass proxy.

Contribution

It introduces a gravity-based selection method for galaxy clusters and reveals a remarkably tight mass-richness relation, contrasting with the ambiguous Y-richness trend.

Findings

Richness correlates tightly with mass (scatter ~0.05 dex).

No outliers in the mass-richness relation, even with projection effects.

Unclear trend between Compton Y and richness in the sample.

Abstract

This paper, the third in a series, investigates the scaling relations between optical richness, weak-lensing mass, and Compton $Y$ for a sample of galaxy clusters selected purely by the effect of their gravitational potential on the shapes of background galaxies. This selection method is uncommon, as most cluster samples in the literature are selected based on signals originating from cluster baryons. We analize a complete sample of 13 gravity-selected clusters at intermediate redshifts (with $0.12 \leq z_{phot} \leq 0.40$) with weak-lensing signal-to-noise ratios exceeding 7. We measured cluster richness by counting red-sequence galaxies, identifying two cases of line-of-sight projections in the process, later confirmed by spectroscopic data. Both clusters are sufficiently separated in redshift that contamination in richness can be straighforwardly dealt because the two red sequences do not blend each other. We find an exceptionally tight richness--mass relation using our red-sequence-based richness estimator, with a scatter of just $\sim0.05$ dex, smaller than the intrinsic scatter of Compton Y with mass for the same sample. The lower scatter highlights the effectiveness of richness compared to Compton $Y$. No outliers are found in the richness-mass scaling, whether or not one cluster with a mass likely affected by projection effects is included in the sample. In the Compton $Y$-richness plane, the data do not delineate a clear trend. The limited sample size is not the sole reason for the unclear relation between Compton $Y$ and richness, since the same sample, with identical richness values, exhibits a highly significant and tight mass-richness correlation.