The SRG/eROSITA All-Sky Survey. Optical Identification and Properties of Galaxy Clusters and Groups in the Western Galactic Hemisphere

The first SRG/ All-Sky Survey ( 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 we measured individual photometric properties (redshift $z_ richness lambda , optical center, and BCG position) for 12\,000 clusters over a sky area of 13\,116\,deg$^2$, augmented by 247 cases identified by matching the candidates with known clusters from the literature. The median redshift of the identified sample is $z=0.31$, with 10 of the clusters at $z>0.72$. The photometric redshifts have an accuracy of $ z/(1+z) for $0.05 and optical completeness >95 for $z>0.05$. For these and further quality assessments of the 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 24\,069\,deg$^2$. Using a combination of these cluster samples, we investigated the velocity dispersion-richness relation, finding that it scales with richness as $ norm with an intrinsic scatter of $ in The primary product of our work is the identified cluster catalog with high purity and a well-defined X-ray selection process, opening the path for precise cosmological analyses presented in companion papers.