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Astron. Astrophys. 362, L45-L48 (2000)

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Letter to the Editor

EROs and the formation epoch of field ellipticals

E. Daddi 1, A. Cimatti 2 and A. Renzini 3

1 Università degli Studi di Firenze, Dipartimento di Astronomia e Scienza dello Spazio, Largo E. Fermi 5, 50125 Firenze, Italy
2 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
3 European Southern Observatory, 85748 Garching, Germany

Received 14 September 2000 / Accepted 4 October 2000


A comparison is presented between the observed surface density of extremely red objects (EROs) and the densities of red high-z elliptical galaxies predicted by Pure Luminosity Evolution (PLE) and by some CDM hierarchical models. The analysis is based on the widest field survey for EROs available to date (covering in total [FORMULA]850 arcmin2), and it takes into account the effects of ERO clustering. Good agreement is found between the observed surface densities of EROs and those predicted by PLE models for ellipticals selected using the same color and luminosity thresholds. Since there is evidence that the bulk of EROs are passively evolving ellipticals, this result implies that most field ellipticals were fully assembled at least by [FORMULA]. Existing hierarchical models predict instead a surface density of red ellipticals that is much smaller than that of the EROs.

Key words: galaxies: elliptical and lenticular, cD – galaxies: evolution – galaxies: formation

Send offprint requests to: edaddi@arcetri.astro.it

© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000