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

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 measure individual photometric properties (redshift z_λ, richness λ, optical center, and BCG position) for 12,000 eRASS1 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 eRASS1 sample is z=0.31, with 10 photometric redshifts have an accuracy of δ z/(1+z)<0.005 for 0.050.05. For these and further quality assessments of the eRASS1 identified catalog, we apply 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 investigate the velocity dispersion-richness relation, finding log(λ)=2.401×log(σ)-5.074 with an intrinsic scatter of 0.10±0.01 dex. Our main result is the identified eRASS1 cluster catalog with a high purity and a well-defined X-ray selection process, enabling precise cosmological analyses presented in companion papers.