EROs and the formation epoch of field ellipticals

A comparison is presented between the observed surface density of extremely red objects (EROs) and the densities of red high-z elliptical galaxies pre dicted by Pure Luminosity Evolution (PLE) and by some CDM hierarchical mode ls. The analysis is based on the widest field survey for EROs available to date (covering in total similar to 850 arcmin(2)), and it takes into accoun t the effects of ERO clustering. Good agreement is found between the observ ed surface densities of EROs and those predicted by PLE models for elliptic als selected using the same color and luminosity thresholds. Since there is evidence that the bulk of EROs are passively evolving ellipticals, this re sult implies that most field ellipticals were fully assembled at least by z similar to 2.5. Existing hierarchical models predict instead a surface den sity of red ellipticals that is much smaller than that of the EROs.