High - z massive galaxies in the HDF - South

A census of massive galaxies at redshift increasingly higher than z ∼ 1 may provide strong constraints on the history of mass assembly and of star formation. Here we report the analysis of three galaxies selected in the Hubble Deep Field South at Ks≤ 22 on the basis of their unusually red near-IR color J-K≥ 3. We have used population synthesis models to constrain their redshifts and their stellar masses. One galaxy (HDFS-1269) is at redshift zphot ≃ 2.4 while the other two (HDFS-822 and HDFS-850) are at zphot ≃ 2.9 − 3.0. All three galaxies have already assembled a stellar mass of about 10 M⊙ at the observed redshift placing the possible merging event of their formation at z∼3.5. The inferred mass weighted age of their stellar populations implies that the bulk of the stars formed at zf > 3.5. The resulting co-moving density of Mstars∼>10 11 M⊙ galaxies at 〈z〉 ≃ 2.7 is ρ = 1.2±0.7×10 Mpc, about a factor two higher than the predictions of hierarchical models. The comparison with the local density of galaxies implies that the three galaxies must have already formed most of their stellar mass and that they cannot follow an evolution significantly different from a passive aging. The comparison with the density of local L≥L early types (passively evolved galaxies) suggests that their co-moving density cannot decrease by more than a factor 2.5-3 from z = 0 to z ≃ 3 suggesting that at least 40% of the stellar mass content of bright (L≥L) local early type galaxies was already in place at z > 2.5.