An Optical Group Catalogue to Z = 1 from the Zcosmos 10k Sample

We present a galaxy group catalogue spanning the redshift range 0.1 . z . 1 in the ∼ 1.7 deg COSMOS field, based on the first ∼ 10, 000 zCOSMOS spectra. The performance of both the Friendsof-Friends (FOF) and Voronoi-Delaunay-Method (VDM) approaches to group identification has been extensively explored and compared using realistic mock catalogues. 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 catalogue, in terms of completeness and purity, is obtained by combining the independently created FOF and VDM catalogues. The final completeness and purity of this catalogue, both in terms of the groups and of individual members, compares favorably with recent results in the literature. The current group catalogue contains 102 groups with N ≥ 5 spectroscopically confirmed members, with a further ∼ 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 catalogues, with quantifiable uncertainties. The fraction of zCOSMOS galaxies in groups is about 25% at low redshift and decreases toward ∼ 15% at z ∼ 0.8. The zCOSMOS group catalogue is broadly consistent with that expected from the semi-analytic evolution model underlying the mock catalogues. Not least, we show that the number density of groups with a given intrinsic richness increases from redshift z ∼ 0.8 to the present, consistent with the hierarchical growth of structure. Subject headings: catalogs — galaxies: clusters: general — galaxies: high-redshift — methods: data analysis 1 European Southern Observatory (ESO), Large Program 175.A-0839 2 Institute of Astronomy, ETH Zurich, Switzerland 3 Max-Planck-Institut für extraterrestrische Physik, D-84571 Garching, Germany 4 European Southern Observatory, Karl-SchwarzschildStrasse 2, Garching, D-85748, Germany 5 INAF Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127, Bologna, Italy 6 Laboratoire dAstrophysique de Toulouse-Tarbes, Universit de Toulouse, CNRS, 14 avenue Edouard Belin, F-31400 Toulouse, France 7 INAF Osservatorio Astronomico di Brera, Milan, Italy 8 Laboratoire d’Astrophysique de Marseille, Marseille, France 9 INAF IASF Milano, Milan, Italy 10 Dipartimento di Astronomia, Universitá di Bologna, via Ranzani 1, I-40127, Bologna, Italy 11 Centre de Physique Theorique, Marseille, Marseille, France 12 Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98 bis Boulevard Arago, F-75014 Paris, France 13 Dipartimento di Astronomia, Universita di Padova, Padova, Italy 14 INAF, Osservatorio di Roma, Monteporzio Catone (RM), Italy 15 ELSA Marie Curie Postdoctoral Fellow, INAF Osservatorio Astronomico di Torino, 10025 Pino Torinese, Italy 16 Universitats-Sternwarte, Scheinerstrasse 1, D-81679 Muenchen, Germany 17 Argelander-Institut für Astronomie, Auf dem Hügel 71, D-53121 Bonn, Germany 18 LBNL & BCCP, University of California, Berkeley, CA, 94720 19 Max Planck Institute of Astrophysics, Karl-Schwarzschild