Optical Redshift and Richness Estimates for Galaxy Clusters Selected with the Sunyaev-Zel'dovich Effect from 2008 South Pole Telescope Observations

TL;DR

This paper reports on optical redshift and richness measurements for galaxy clusters detected via the Sunyaev-Zel'dovich effect, demonstrating the effectiveness of optical data in confirming and characterizing these clusters for cosmological studies.

Contribution

It introduces a method to estimate cluster redshifts and masses using optical richness from SZE-selected clusters, validated with spectroscopic data and scaling relations.

Findings

Optical richness correlates well with SZE-derived masses.

Redshift estimates from red-sequence colors have 2% RMS accuracy for z<1.

Clusters span redshifts from 0.15 to >1, median 0.74.

Abstract

We present redshifts and optical richness properties of 21 galaxy clusters uniformly selected by their Sunyaev-Zel'dovich signature. These clusters, plus an additional, unconfirmed candidate, were detected in a 178 square-degree area surveyed by the South Pole Telescope in 2008. Using griz imaging from the Blanco Cosmology Survey and from pointed Magellan telescope observations, as well as spectroscopy using Magellan facilities, we confirm the existence of clustered red-sequence galaxies, report red-sequence photometric redshifts, present spectroscopic redshifts for a subsample, and derive R_200 radii and M_200 masses from optical richness. The clusters span redshifts from 0.15 to greater than 1, with a median redshift of 0.74; three clusters are estimated to be at z > 1. Redshifts inferred from mean red-sequence colors exhibit 2% RMS scatter in sigma_z/(1+z) with respect to the spectroscopic subsample for z < 1. We show that M_200 cluster masses derived from optical richness correlate with masses derived from South Pole Telescope data and agree with previously derived scaling relations to within the uncertainties. Optical and infrared imaging is an efficient means of cluster identification and redshift estimation in large Sunyaev-Zel'dovich surveys, and exploiting the same data for richness measurements, as we have done, will be useful for constraining cluster masses and radii for large samples in cosmological analysis.