A deeper view of extremely red galaxies: the redshift distribution in the GOODS/CDFS ISAAC field

Abstract

We have analysed five-epoch Great Observatories Origins Deep Survey (GOODS) HST-ACS B, V, I₇₇₅ and z data sets (v. 1.0 release), in conjunction with existing VLT-ISAAC imaging in the J, H and K_s bands, to derive estimated redshifts for the sample of 198 extremely red galaxies (ERGs) with K_s < 22 (Vega) and (I₇₇₅-K_s) > 3.92 selected by Roche, Dunlop & Almaini (2003) from 50.4 arcmin² of the GOODS/Chandra Deep Field South. 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 (~3 × 10⁹ M_solar to ~3 × 10¹¹ M_solar). Our results show that the dust-corrected red envelope of galaxy evolution is well modelled by a starburst at redshift z_f= 5 followed thereafter by passive evolution. 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 K_s-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 ρ_c= (6.1 +/- 1.9) × 10^-5 Mpc^-3 and ρ_c= (2.1 +/- 1.1) × 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 two ERGs which have a high probability of lying at z_phot > 4. We discuss the advantages of multicolour 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.