Letter to the Editor

The manifold spectra and morphologies of EROs^*

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio, 2, 35122 Padova, Italy
⁴ INAF – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio, 5, 35122 Padova, Italy
⁵ National Radio Astronomy Observatory, Charlottesville, VA 22903, USA
⁶ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁷ INAF – Osservatorio Astronomico di Roma, via dell'Osservatorio 2, Monteporzio, Italy

Corresponding author: A. Cimatti, cimatti@arcetri.astro.it

Received: 3 July 2003
Accepted: 13 October 2003

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 and . 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 (∼%), spiral-like (~%) and irregular systems (~%). 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  yr^-1. The colors, redshifts and masses of the E/S0 galaxy subsample imply a minimum formation redshift . With this z_f there is enough time to have old and massive stellar spheroids already assembled at . We verify that the vs. color diagram is efficient in segregating old and dusty-star-forming EROs.