A deeper view of Extremely Red Galaxies: the redshift distribution in the GOODS/CDFS ISAAC field

Abstract

We have analysed 5-epoch GOODS HST-ACS B, V, I_775 and z datasets (V1.0 release), in conjunction with existing VLT-ISAAC imaging in the J, H and Ks bands, to derive estimated redshifts for the sample of 198 Extremely Red Galaxies (ERGs) with Ks<22 (Vega) and I_775-Ks>3.92 selected by Roche et al.(2003) from 50.4 sq.arcmin of the GOODS/CDFS field. We find that, at this depth, the ERG population spans the redshift range 0.5<z_phot<4.75 and over two decades in mass (~ 3x10^9 M_sun to ~3x10^11 M_sun). We explore the evolution of the ERG luminosity function (LF) from redshifts <z_phot>=1.0 to <z_phot>=2.5 and compare it with the global Ks-band LF at redshifts 1<z_phot<2. We find that the bright end of the ERG LF does not decrease from redshifts <z_phot>=2.0 to <z_phot>=2.5 and we connect this fact with the presence of progenitors of the local L>L* population at redshifts z_phot>2. We determine lower limits of rho_c=(6.1 +/- 1.9) x 10^{-5} Mpc^{-3} and rho_c=(2.1 +/- 1.1) x 10^{-5}Mpc^{-3} on the comoving densities of progenitors of local massive galaxies already assembled at redshifts <z_phot>=2.5 and <z_phot>=3.5, respectively. We have investigated the existence of high-redshift Lyman break galaxies massive enough to be included in this ERG sample. Out of an initial list of 12 potential very high redshift candidates, we have identified 2 ERGs which have a high probability of lying at z_phot>4. We discuss the advantages of multi-colour to single-colour selection techniques in obtaining reliable lists of very high-redshift candidate sources, and present revised lower redshift estimates for sources previously claimed as potential z> 5 dropouts in recent studies.(abridged)