AN OPTICAL GROUP CATALOG TOz= 1 FROM THE zCOSMOS 10 k SAMPLE

We present a galaxy group catalog spanning the redshift range 0.1 less than or similar to z less than or similar to 1 in the similar to 1.7 deg(2) COSMOS field, based on the first similar to 10,000 zCOSMOS spectra. The performance of both the Friends-of-Friends (FOF) and Voronoi Delaunay method (VDM) approaches to group identification has been extensively explored and compared using realistic mock catalogs. We find that the performance improves substantially if groups are found by progressively optimizing the group-finding parameters for successively smaller groups, and that the highest fidelity catalog, in terms of completeness and purity, is obtained by combining the independently created FOF and VDM catalogs. The final completeness and purity of this catalog, both in terms of the groups and of individual members, compares favorably with recent results in the literature. The current group catalog contains 102 groups with N >= 5 spectroscopically confirmed members, with a further similar to 700 groups with 2 <= N <= 4. Most of the groups can be assigned a velocity dispersion and a dark-matter mass derived from the mock catalogs, with quantifiable uncertainties. The fraction of zCOSMOS galaxies in groups is about 25% at low redshift and decreases toward similar to 15% at z similar to 0.8. The zCOSMOS group catalog is broadly consistent with that expected from the semianalytic evolution model underlying the mock catalogs. Not least, we show that the number density of groups with a given intrinsic richness increases from redshift z similar to 0.8 to the present, consistent with the hierarchical growth of structure.