Galaxy Clusters Identified from the SDSS DR6 and Their Properties

Clusters of galaxies in most of the previous catalogs have redshifts z <= 0.3. Using the photometric redshifts of galaxies from the Sloan Digital Sky Survey Data Release 6 (SDSS DR6), we identify 39,716 clusters in the redshift range 0.05 < z < 0.6 with more than eight luminous (M_r <= -21) member galaxies. Cluster redshifts are estimated accurately with an uncertainty of less than 0.022. The contamination rate of member galaxies is found to be roughly 20%, and the completeness of member galaxy detection reaches ~90%. Monte Carlo simulations show that the cluster detection rate is more than 90% for massive (M ₂₀₀ > 2 × 10¹⁴ M _sun) clusters of z <= 0.42. The false detection rate is ~5%. We obtain the richness, the summed luminosity, and the gross galaxy number within the determined radius for identified clusters. They are tightly related to the X-ray luminosity and temperature of clusters. Cluster mass is related to the richness and summed luminosity with M ₂₀₀ vprop R ^1.90±0.04 and M ₂₀₀ vprop L ^1.64±0.03 _r , respectively. In addition, 790 new candidates of X-ray clusters are found by cross-identification of our clusters with the source list of the ROSAT X-ray survey.