GMASS ultradeep spectroscopy of galaxies at z ∼ 2 -: III.: The emergence of the color bimodality at z ∼ 2

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 1.4 < z < 3. 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 mu m) = 23. Results. We find that the distribution of galaxies in the (U - B) color vs. stellar mass plane is bimodal up to at least redshift z = 2. We define a "mass complete" sample of galaxies residing on the red sequence, selecting objects with log(M/M-circle dot) > 10.1, 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 z < 0.5 down to similar to 50% at redshift z = 2. At z > 2 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 z > 2 are passively evolving, with the bulk of their stars formed at redshift z > 3.