The VIMOS-VLT deep survey^*

Color bimodality and the mix of galaxy populations up to z ~ 2

¹ INAF – IASF Milano, via Bassini 15, 20133 Milano, Italy e-mail: paolo@lambrate.inaf.it
² INAF – Osservatorio Astronomico di Brera, via Brera 28, Milano, Italy
³ Laboratoire d'Astrophysique de Marseille, UMR 6110 CNRS-Université de Provence, BP 8, 13376 Marseille Cedex 12, France
⁴ Institute for Astronomy, 2680 Woodlawn Dr., University of Hawaii, Honolulu, 96822 Hawaii, USA
⁵ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁶ Laboratoire d'Astrophysique de l'Observatoire Midi-Pyrénées (UMR 5572), 14 avenue E. Belin, 31400 Toulouse, France
⁷ Centro de Astrofisica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁸ Institut d'Astrophysique de Paris, UMR 7095, 98bis Bd. Arago, 75014 Paris, France
⁹ INAF – IRA, via Gobetti 101, 40129 Bologna, Italy
¹⁰ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
¹¹ School of Physics & Astronomy, University of Nottingham, University Park, Nottingham NG72RD, UK
¹² Astrophysical Institute Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹³ Observatoire de Paris, LERMA, 61 avenue de l'Observatoire, 75014 Paris, France
¹⁴ Università di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
¹⁵ Centre de Physique Théorique, UMR 6207 CNRS-Université de Provence, 13288 Marseille, France
¹⁶ Integral Science Data Centre, Ch. d'Écogia 16, 1290 Versoix, Switzerland
¹⁷ Geneva Observatory, Ch. des Maillettes 51, 1290 Sauverny, Switzerland
¹⁸ INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
¹⁹ Università di Milano-Bicocca, Dipartimento di Fisica – Piazza delle Scienze 3, 20126 Milano, Italy

Received: 30 June 2006
Accepted: 11 January 2007

Abstract

Aims. In this paper we discuss the mix of star-forming and passive galaxies up to z ~ 2, based on the first epoch VIMOS-VLT Deep Survey (VVDS) data.

Methods. We compute rest-frame magnitudes and colors and analyse the color-magnitude relation and the color distributions. We also use the multi-band VVDS photometric data and spectral templates fitting to derive multi-color galaxy types. Using our spectroscopic dataset we separate galaxies based on a star-formation activity indicator derived combining the equivalent width of the [OII] emission line and the strength of the D_n(4000) continuum break.

Results. In agreement with previous works we find that the global galaxy rest-frame color distribution follows a bimodal distribution at z ≤ 1, and we establish that this bimodality holds up to at least . The details of the rest-frame color distribution depend however on redshift and on galaxy luminosity, with faint galaxies being bluer than the luminous ones over the whole redshift range covered by our data, and with galaxies becoming bluer as redshift increases. This latter blueing trend does not depend, to a first approximation, on galaxy luminosity. The comparison between the spectral classification and the rest-frame colors shows that about 35-40% of the red objects are in fact star forming galaxies. Hence we conclude that the red sequence cannot be used to effectively isolate a sample of purely passively evolving objects within a cosmological survey. We show how multi-color galaxy types have a slightly higher efficiency than rest-frame color in isolating the passive, non star-forming galaxies within the VVDS sample. Connected to these results is also the finding that the color-magnitude relations derived for the color and for the spectroscopically selected early-type galaxies have remarkably similar properties, with the contaminating star-forming galaxies within the red sequence objects introducing no significant offset in the rest frame colors. Therefore the average color of the red objects does not appear to be a very sensitive indicator for measuring the evolution of the early-type galaxy population.