The VIMOS-VLT deep survey
Color bimodality and the mix of galaxy populations up to z ∼2

P. Franzetti, M. Scodeggio, B. Garilli, D. Vergani, D. Maccagni, L. Guzzo, L. Tresse, O. Ilbert, F. Lamareille, T. Contini, O. Le Fèvre, G. Zamorani, J. Brinchmann, S. Charlot, D. Bottini, V. Le Brun, J.-P. Picat, R. Scaramella, G. Vettolani, A. Zanichelli, C. Adami, S. Arnouts, S. Bardelli, M. Bolzonella, A. Cappi, P. Ciliegi, S. Foucaud, I. Gavignaud, A. Iovino, H. J. McCracken, B. Marano, C. Marinoni, A. Mazure, B. Meneux, R. Merighi, S. Paltani, R. Pellò, A. Pollo, L. Pozzetti, M. Radovich, E. Zucca, O. Cucciati, C. J. Walcher

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 Dn(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 z = 1.5. 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.

Keywords
galaxies: evolution – galaxies: fundamental parameters – galaxies: photometry

Astronomy and Astrophysics
Volume 465, Page 711
April 2007

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