In 2019, the eROSITA telescope on board of the Russian-German satellite Spectrum-Roentgen-Gamma (SRG), has started to perform a deep all-sky X-ray survey with the aim of identifying ~100000 clusters and groups over the course of four years. As part of its performance verification phase a ~140deg^2^ survey called eROSITA Final Equatorial-Depth Survey (eFEDS) was performed. With a depth typical of the all-sky survey after four years, it allows tests of tools and methods as well as improved predictions for the all-sky survey. As part of this effort, a catalog of 542 X-ray extent selected galaxy group and cluster candidates was constructed. In this paper we present the optical follow-up with the aim of providing redshifts and cluster confirmation for the full sample. Further we aim to provide additional information on cluster dynamical state, cluster richness and cluster optical center. Finally we aim to evaluate the impact of optical cluster confirmation on the purity and completeness of the X-ray selected sample. We use optical imaging data from the Hyper Suprime-Cam Subaru Strategic Program and from the Legacy Survey to identify optical counterparts to the X-ray detected cluster candidates. We make use of the red sequence based cluster redshift and confirmation tool MCMF as well as the optical cluster finder CAMIRA to derive cluster redshifts and richnesses. MCMF provided probabilities of an optical structure being a chance super position with the X-ray candidate is used to identify the best optical counter part as well as to confirm an X-ray candidate as a cluster. The impact of this confirmation process on catalog purity and completeness is estimated using optical to X-ray scaling relations as well as simulations. The resulting catalog is further matched with public group and cluster catalogs. Optical estimators of cluster dynamical state are constructed based on density maps of the red sequence galaxies at the cluster redshift. While providing redshift estimates for all 542 candidates, we construct an optically confirmed sample of 477 clusters and groups with a residual contamination of 6%. Of these, 470 (98.5%) are confirmed using MCMF, and 7 systems are added through cross matching with spectroscopic group catalogs. Using observable to observable scaling and the applied confirmation threshold, we predict 8+/-2 real systems have been excluded with the MCMF cut required to build this low contamination sample. This number is in good agreement with the 7 systems found through cross matching that were not confirmed with MCMF. The predicted redshift and mass distribution of this catalog is in good agreement with simulations. Thus, we expect those 477 systems to include >99% of all true clusters in the candidate list. Using an MCMF independent method, we confirm the catalog contamination of the confirmed subsample to be 6+/-3%. Applying the same method to the full candidate list yields 17+/-3%, consistent with estimates coming from the fraction of confirmed systems of ~17% and expectations from simulations of ~20%. We further present a sample of merging cluster candidates based on the derived estimators of cluster dynamical state.

Cone search capability for table J/A+A/661/A4/catalog (Cluster catalog with MCMF entries)