Old massive ellipticals and S0 in the Hubble deep field vanish from view at z > 1.3: Possible solutions of the enigma

We have investigated the properties of a bright K-band selected sample of early-type galaxies in the Hubble Deep Field, as representative of the field galaxy population. This dataset, based on public archives from HST and from deep observations at Hawaii and Kitt-Peak, is unique as for the morphological information on faint high-z sources, and for complete photometric and spectroscopic coverage. The majority of bright early-type galaxies in this field are found at redshifts z ≤ 1.3 to share common properties with those of high-z cluster samples, as for the age and mass of the dominant stellar population – which are found to be as old as 3-5 Gyr and as massive as 10 M⊙ already at z ≃ 1. The bright end of the E/S0 population is already in place by that cosmic epoch, with space densities, masses and luminosities consistent with those of the local field population. There is no evidence of a steepening of the mass function from z=0 to z=1, as inferred by some authors from analyses of optically-selected samples and favoured by hierarchical clustering models forming most of the E/S0s at z 1.3, which would be expected as clearly detectable above the flux limits, given the aged properties of the lower redshift counterparts. So, something hide them at high redshifts. Merging could be an explanation, but it would require that already formed and old stellar systems would assemble on short timescales of Dipartimento di Astronomia, Vicolo Osservatorio 5, I-35122, Padova, Italy. E-mail: Franceschini@astrpd.pd.astro.it International School for Advanced Studies, Via Beirut, I34014 Trieste, Italy Osservatorio Astronomico, Vicolo dell’Osservatorio 5, I35122 Padova, Italy.