The eROSITA Final Equatorial-Depth Survey (eFEDS): Optical confirmation,
  redshifts, and properties of the cluster and group catalog
  The eROSITA Final Equatorial-Depth Survey (eFEDS), covering ~140 square
degrees, was performed as part of the performance verification phase of the
eROSITA telescope on board of the Russian-German satellite
Spectrum-Roentgen-Gamma (SRG). In this paper we present the optical follow-up
of 542 X-ray extent selected galaxy group and cluster candidates providing
redshifts and cluster confirmation for the full sample. We use optical imaging
data from the Hyper Suprime-Cam Subaru Strategic Program and from the Legacy
Survey to run the cluster redshift and confirmation tool MCMF as well as the
optical cluster finder CAMIRA at the location of the X-ray candidates. 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
recovered through cross matching. 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% and find 17 +/- 3% contamination for the full X-ray sample. The
estimated contamination of the fulls sample is in agreement with MCMF dependent
estimate of ~17% and the expectation from dedicated X-ray simulations of ~20%.
We further present a sample of optically selected merging cluster candidates.
