The VIMOS-VLT deep survey: the group catalogue^*

¹ INAF-Osservatorio Astronomico di Brera, via Brera 28, 20021 Milan, Italy e-mail: cucciati@brera.mi.astro.it
² Laboratoire d'Astrophysique de Marseille, Université de Provence, CNRS, 38 rue Frederic Joliot-Curie, 13388 Marseille Cedex 13, France
³ Centre de Physique Théorique, UMR 6207, CNRS-Université de Provence, 13288 Marseille, France
⁴ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁵ IASF-INAF, via Bassini 15, 20133 Milano, Italy
⁶ Institute of Astro- and Particle Physics, Leopold-Franzens-University Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria
⁷ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁸ Université de Lyon, 69003 Lyon, France; Université Lyon 1, Observatoire de Lyon, 9 avenue Charles André, Saint-Genis Laval, 69230, France; CNRS, UMR 5574, Centre de Recherche Astrophysique de Lyon; École Normale Supérieure de Lyon, 69007 Lyon, France
⁹ Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching bei München, Germany
¹⁰ Universitätssternwarte München, Scheinerstrasse 1, 81679 München, Germany
¹¹ IRA-INAF, via Gobetti 101, 40129 Bologna, Italy
¹² Canada France Hawaii Telescope corporation, Mamalahoa Hwy, Kamuela, 96743, USA
¹³ Max Planck Institut für Astrophysik, 85741 Garching, Germany
¹⁴ Institut d'Astrophysique de Paris, UMR 7095, 98 bis Bvd Arago, 75014 Paris, France
¹⁵ Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 Av. E. Belin, 31400 Toulouse, France
¹⁶ School of Physics & Astronomy, University of Nottingham, University Park, Nottingham, NG72RD, UK
¹⁷ Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹⁸ Observatoire de Paris, LERMA, 61 Avenue de l'Observatoire, 75014 Paris, France
¹⁹ Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
²⁰ Integral Science Data Centre, ch. d'Écogia 16, 1290 Versoix, Switzerland
²¹ Geneva Observatory, ch. des Maillettes 51, 1290 Sauverny, Switzerland
²² The Andrzej Soltan Institute for Nuclear Studies, ul. Hoza 69, 00-681 Warszawa, Poland
²³ Astronomical Observatory of the Jagiellonian University, ul Orla 171, 30-244 Kraków, Poland
²⁴ Instituto de Astrofísica de Andalucía – CSIC. Apdo. de correos 3004, 18080 Granada, Spain

Received: 11 February 2009
Accepted: 7 May 2010

Abstract

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(σ), 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 ~60% and a purity of ~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 (~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.