The VIMOS-VLT deep survey: the group catalogue

Aims. We present a homogeneous and complete catalogue of optical galaxy groups identified in the purely flux-limited (17.5 <= I-AB <= 24.0) VIMOS-VLT deep redshift Survey (VVDS). Methods. We use mock catalogues extracted from the Millennium Simulation, to correct for potential systematics that might affect the overall distribution as well as the individual properties of the identified systems. Simulated samples allow us to forecast the number and properties of groups that can be potentially found in a survey with VVDS-like selection functions. We use them to correct for the expected incompleteness and, to asses in addition, how well galaxy redshifts trace the line-of-sight velocity dispersion of the underlying mass overdensity. In particular, on these mock catalogues we train the adopted group-finding technique i.e., the Voronoi-Delaunay Method (VDM). The goal is to fine-tune its free parameters, recover in a robust and unbiased way the redshift and velocity dispersion distributions of groups (n(z) and n(sigma), respectively), and maximize, at the same time, the level of completeness and purity of the group catalogue. Results. We identify 318 VVDS groups with at least 2 members in the range 0.2 <= z <= 1.0, among which 144 (/30) with at least 3 (/5) members. The sample has an overall completeness of similar to 60% and a purity of similar to 50%. Nearly 45% of the groups with at least 3 members are still recovered if we run the algorithm with a particular parameter set that maximizes the purity (similar to 75%) of the resulting catalogue. We use the group sample to explore the redshift evolution of the fraction f(b) of blue galaxies (U - B <= 1) in the redshift range 0.2 <= z <= 1. We find that the fraction of blue galaxies is significantly lower in groups than in the global population (i.e. in the whole ensemble of galaxies irrespective of their environment). Both of these quantities increase with redshift, the fraction of blue galaxies in groups exhibiting a marginally significant steeper increase. We also investigate the dependence of f(b) on group richness: not only we confirm that, at any redshift, the blue fraction decreases in systems with increasing richness, but we find that this result continues to hold towards fainter luminosities.