Velocity Dispersions of Clusters in the Dark Energy Survey Y3 redMaPPer Catalog

TL;DR

This study measures galaxy cluster velocity dispersions from DES Y3 data to evaluate cluster selection, richness estimates, and the impact of projection effects, revealing a bimodal distribution and high-velocity dispersion outliers.

Contribution

It provides the first detailed analysis of velocity dispersions in DES Y3 redMaPPer clusters, highlighting the significance of projection effects and bimodal distributions in cluster properties.

Findings

Approximately 22% of low-richness clusters have high velocity dispersions.

Over half of the low-richness clusters at z>0.5 are velocity dispersion outliers.

Projection effects significantly influence redMaPPer cluster selection, especially at higher redshifts.

Abstract

We measure the velocity dispersions of clusters of galaxies selected by the redMaPPer algorithm in the first three years of data from the Dark Energy Survey (DES), allowing us to probe cluster selection and richness estimation, $\lambda$, in light of cluster dynamics. Our sample consists of 126 clusters with sufficient spectroscopy for individual velocity dispersion estimates. We examine the correlations between cluster velocity dispersion, richness, X-ray temperature and luminosity as well as central galaxy velocity offsets. The velocity dispersion-richness relation exhibits a bimodal distribution. The majority of clusters follow scaling relations between velocity dispersion, richness, and X-ray properties similar to those found for previous samples; however, there is a significant population of clusters with velocity dispersions which are high for their richness. These clusters account for roughly 22\% of the $\lambda < 70$ systems in our sample, but more than half (55\%) of $\lambda < 70$ clusters at $z>0.5$. A couple of these systems are hot and X-ray bright as expected for massive clusters with richnesses that appear to have been underestimated, but most appear to have high velocity dispersions for their X-ray properties likely due to line-of-sight structure. These results suggest that projection effects contribute significantly to redMaPPer selection, particularly at higher redshifts and lower richnesses. The redMaPPer determined richnesses for the velocity dispersion outliers are consistent with their X-ray properties, but several are X-ray undetected and deeper data is needed to understand their nature.