EDP Sciences
Free access
Volume 412, Number 1, December II 2003
Page(s) L1 - L5
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20031601

A&A 412, L1-L5 (2003)
DOI: 10.1051/0004-6361:20031601


The manifold spectra and morphologies of EROs

A. Cimatti1, E. Daddi2, P. Cassata3, E. Pignatelli4, G. Fasano4, J. Vernet1, E. Fomalont5, K. Kellermann5, G. Zamorani6, M. Mignoli6, L. Pozzetti6, A. Renzini2, S. di Serego Alighieri1, A. Franceschini3, E. Giallongo7 and A. Fontana7

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

(Received 3 July 2003 / Accepted 13 October 2003)

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 $R-K_{\rm s}>5$ and $K_{\rm s}<20$. 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 ( $\sim $ 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  $M_{\odot}$ yr -1. The colors, redshifts and masses of the E/S0 galaxy subsample imply a minimum formation redshift $z_{\rm f}\sim 2$. With this $z_{\rm f}$ there is enough time to have old and massive stellar spheroids already assembled at $z\sim 1$. We verify that the $R-K_{\rm s}$ vs. $J-K_{\rm s}$ color diagram is efficient in segregating old and dusty-star-forming EROs.

Key words: galaxies: evolution -- galaxies: elliptical and lenticular, cD -- galaxies: starbust -- galaxies: formation

Offprint request: A. Cimatti, cimatti@arcetri.astro.it

© ESO 2003