Letter to the Editor

GMASS ultradeep spectroscopy of galaxies at z

III. The emergence of the color bimodality at z

¹ Laboratoire d'Astrophysique de Marseille, OAMP, UMR6110, CNRS-Université de Provence Aix-Marseille I, BP 8, 13376 Marseille Cedex 12, France e-mail: paolo.cassata@oamp.fr
² Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Max-Planck-Institut für Astronomie, Königstul 17, 69117 Heidelberg, Germany
⁴ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy
⁵ INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁶ CEA-Saclay, DSM/DAPNIA/Service d'Astrophysique, 91191 Gif-sur-Yvette Cedex, France
⁷ Max Planck Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching bei München, Germany
⁸ NOAO-Tucson, 950 North Cherry Avenue, Tucson, AZ 85719, USA
⁹ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁰ INAF - Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
¹¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany

Received: 1 April 2008
Accepted: 8 April 2008

Abstract

Aims. The aim of this work is to study the evolution of the rest-frame color distribution of galaxies with the redshift, in particular in the critical interval .

Methods. We combined ultradeep spectroscopy from the GMASS project (Galaxy Mass Assembly ultradeep Spectroscopic Survey) with GOODS multi-band photometry (from optical to mid-infrared) to study a sample of 1021 galaxies up to m(4.5 μm) = 23.

Results. We find that the distribution of galaxies in the  color vs. stellar mass plane is bimodal up to at least redshift . We define a “mass complete” sample of galaxies residing on the red sequence, selecting objects with , and we study their morphological and spectro-photometric properties. We show that the contribution to this sample of early-type galaxies, defined as galaxies with a spheroidal morphology and no star formation, decreases from 60-70% at 0.5 down to ~50% at redshift . At we still find red galaxies in the mass complete sample, even if the bimodality is not seen any more. About 25% of these red galaxies at are passively evolving, with the bulk of their stars formed at redshift .