An optical group catalogue to z = 1 from the zCOSMOS 10k sample

TL;DR

This paper presents a detailed galaxy group catalogue up to redshift 1 from the zCOSMOS survey, demonstrating improved group-finding methods and analyzing the evolution of galaxy groups over cosmic time.

Contribution

It introduces an optimized method for identifying galaxy groups combining FOF and VDM techniques, resulting in a high-fidelity catalogue with quantified uncertainties.

Findings

The catalogue contains 102 groups with N >= 5 members.

The fraction of galaxies in groups decreases from 25% to 15% from low redshift to z ~ 0.8.

The number density of groups increases over time, consistent with hierarchical structure growth.

Abstract

We present a galaxy group catalogue spanning the redshift range 0.1 <~ z <~ 1 in the ~1.7 deg^2 COSMOS field, based on the first ~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 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.