The red and blue galaxy populations in the GOODS field: evidence for an excess of red dwarfs

¹ INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio (RM), Italy e-mail: salimbeni@mporzio.astro.it
² Dipartimento di Fisica, Universitá di Roma “La Sapienza”, P.le A. Moro 2, 00185 Roma, Italy
³ INAF - Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy

Received: 28 May 2007
Accepted: 18 October 2007

Abstract

Aims. We study the evolution of the galaxy population up to as a function of its colour properties. In particular, luminosity functions and luminosity densities were derived as a function of redshift for the blue/late and red/early populations.

Methods.We use data from the GOODS-MUSIC catalogue, which have typical magnitude limits and for most of the sample. About 8% of the galaxies have spectroscopic redshifts; the remaining have well calibrated photometric redshifts derived from the extremely wide multi-wavelength coverage in 14 bands (from the U band to the Spitzer m band). We have derived a catalogue of galaxies complete in the rest-frame B-band, which has been divided into two subsamples according to their rest-frame  colour (or derived specific star formation rate) properties.

Results.We confirm a bimodality in the  colour and specific star formation rate of the galaxy sample up to . This bimodality is used to compute the luminosity functions of the blue/late and red/early subsamples. The luminosity functions of the blue/late and total samples are well represented by steep Schechter functions evolving in luminosity with increasing redshifts. The volume density of the luminosity functions of the red/early populations decreases with increasing redshift. The shape of the red/early luminosity functions shows an excess of faint red dwarfs with respect to the extrapolation of a flat Schechter function and can be represented by the sum of two Schechter functions. Our model for galaxy formation in the hierarchical clustering scenario, which also includes external feedback due to a diffuse UV background, shows a general broad agreement with the luminosity functions of both populations, the larger discrepancies being present at the faint end for the red population. Hints on the nature of the red dwarf population are given on the basis of their stellar mass and spatial distributions.