The C4 Clustering Algorithm: Clusters of Galaxies in the Sloan Digital Sky Survey

We present the "C4 Cluster Catalog", a new sample of 748 clusters of galaxiesidentified in the spectroscopic sample of the Second Data Release (DR2) of theSloan Digital Sky Survey (SDSS). The C4 cluster--finding algorithm identifiesclusters as overdensities in a seven-dimensional position and color space, thusminimizing projection effects which plagued previous optical clustersselection. The present C4 catalog covers ~2600 square degrees of sky withgroups containing 10 members to massive clusters having over 200 clustermembers with redshifts. We provide cluster properties like sky location, meanredshift, galaxy membership, summed r--band optical luminosity (L_r), velocitydispersion, and measures of substructure. We use new mock galaxy catalogs toinvestigate the sensitivity to the various algorithm parameters, as well as toquantify purity and completeness. These mock catalogs indicate that the C4catalog is ~90% complete and 95% pure above M_200 = 1x10^14 solar masses andwithin 0.03 <=z <= 0.12. The C4 algorithm finds 98% of X-ray identifiedclusters and 90% of Abell clusters within 0.03 <= z <= 0.12. We show that theL_r of a cluster is a more robust estimator of the halo mass (M_200) than theline-of-sight velocity dispersion or the richness of the cluster. L_r. Thefinal SDSS data will provide ~2500 C4 clusters and will represent one of thelargest and most homogeneous samples of local clusters.