The manifold spectra and morphologies of EROs
Cimatti, A.; Daddi, E.; Franceschini, A.; Fontana, A.; Giallongo, E.; di Serego Alighieri, S.; Mignoli, M.; Zamorani, G.; Pozzetti, L.; Cassata, P.; Renzini, A.; Vernet, J.; Fasano, G.; Pignatelli, E.; Fomalont, E.; Kellermann, K.
Italy, Germany, United States
Abstract
Deep VLT optical spectroscopy, HST+ACS (GOODS) imaging and VLA observations are used to unveil the nature of a complete sample of 47 EROs with R-Ks>5 and Ks<20. The spectroscopic redshift completeness is 62%. Morphological classification was derived for each ERO through visual inspection and surface brightness profile fitting. Three main ERO morphological types are found: E/S0 galaxies ( ~ 30-37%), spiral-like ( ~ 24-46%) and irregular systems ( ~ 17-39%). The only ERO detected in the radio is likely to host an obscured AGN. The average radio luminosity of the star-forming EROs undetected in the radio implies star formation rates of the order of ~ 33 Msun yr-1. The colors, redshifts and masses of the E/S0 galaxy subsample imply a minimum formation redshift zf ~2. With this zf there is enough time to have old and massive stellar spheroids already assembled at z ~ 1. We verify that the R-Ks vs. J-Ks color diagram is efficient in segregating old and dusty-star-forming EROs.
Based on observations made at the European Southern Observatory, Paranal, Chile (Program 70.A-0140(B)).