High-z massive galaxies in the Hubble Deep Field South

A census of massive galaxies at redshift increasingly higher than zsimilar to1 may provide strong constraints for the history of mass assembly and star formation. Here we report the analysis of three galaxies selected in the Hubble Deep Field South at Ksless than or equal to22 on the basis of their unusually red near-IR color J-Kgreater than or equal to3. We have used population synthesis models to constrain their redshifts and their stellar masses. One galaxy (HDFS-1269) is at redshift z(phot)similar or equal to2.4 while the other two (HDFS-822 and HDFS-850) are at z(phot)similar or equal to2.9-3.0. All three galaxies have already assembled a stellar mass of about 10(11) M-circle dot at the observed redshift, placing the possible merging event of their formation at zgreater than or similar to3.5. The inferred mass weighted age of their stellar populations implies that the bulk of the stars formed at z(f)>3.5. The resulting co-moving density of M(stars)greater than or equal to10(11) M-circle dot galaxies at similar or equal to2.7 is rho=1.2+/-0.7x10(-4) Mpc(-3), 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 Lgreater than or equal toL* 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 zsimilar or equal to3 suggesting that up to 40% of the stellar mass content of bright (Lgreater than or equal toL*) local early type galaxies was already in place at z>2.5.