The SRG/eROSITA All-Sky Survey

Optical identification and properties of galaxy clusters and groups in the western galactic hemisphere^★

¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: mkluge@mpe.mpg.de
² University-Observatory, Ludwig-Maximilians-University, Scheinerstrasse 1, 81679 Munich, Germany
³ IRAP, Université de Toulouse, CNRS, UPS, CNES, 31028 Toulouse, France
⁴ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁵ Excellence Cluster Origins, Boltzmannstr. 2, 85748 Garching, Germany
⁶ McDonald Observatory, University of Texas at Austin, 2515 Speedway, Stop C1402, Austin, TX 78712, USA
⁷ Department of Astronomy, University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
⁸ Argelander-Institut für Astronomie (AIfA), Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁰ Cerro Tololo Inter-American Observatory, NSF’s National Optical-Infrared Astronomy Research Laboratory, Casilla 603, La Serena, Chile

Received: 20 December 2023
Accepted: 30 April 2024

Abstract

The first SRG/eROSITA All-Sky Survey (eRASS1) provides the largest intracluster medium-selected galaxy cluster and group catalog covering the western Galactic hemisphere. Compared to samples selected purely on X-ray extent, the sample purity can be enhanced by identifying cluster candidates using optical and near-infrared data from the DESI Legacy Imaging Surveys. Using the red-sequence-based cluster finder eROMaPPer, we measured individual photometric properties (redshift z_λ, richness λ, optical center, and BCG position) for 12000 eRASS1 clusters over a sky area of 13 116 deg², augmented by 247 cases identified by matching the candidates with known clusters from the literature. The median redshift of the identified eRASS1 sample is z = 0.31, with 10% of the clusters at z > 0.72. The photometric redshifts have an accuracy of δz/(1 + z) ≲ 0.005 for 0.05 < z < 0.9. Spectroscopic cluster properties (redshift z_spec and velocity dispersion σ) were measured a posteriori for a subsample of 3210 and 1499 eRASS1 clusters, respectively, using an extensive compilation of spectroscopic redshifts of galaxies from the literature. We infer that the primary eRASS1 sample has a purity of 86% and optical completeness >95% for z > 0.05. For these and further quality assessments of the eRASS1 identified catalog, we applied our identification method to a collection of galaxy cluster catalogs in the literature, as well as blindly on the full Legacy Surveys covering 24069 deg². Using a combination of these cluster samples, we investigated the velocity dispersion-richness relation, finding that it scales with richness as log(λ_norm) = 2.401 × log(σ) − 5.074 with an intrinsic scatter of δ_in = 0.10 ± 0.01 dex. The primary product of our work is the identified eRASS1 cluster catalog with high purity and a well-defined X-ray selection process, opening the path for precise cosmological analyses presented in companion papers.