Extremely red objects (EROs, here defined with R-Ks>5) were
discovered serendipitously a decade ago (Elston et al. 1988), and recent
wide-field surveys revealed that they form a substantial
population (Thompson et al. 1999; Daddi et al. 2000a, D00
hereafter; McCarthy et al. 2001). Having the colors expected
for high-z old and passively evolving galaxies, EROs offer
the opportunity to test whether the present-day massive
ellipticals formed at early cosmological times (
2-3) with
a subsequent passive and pure luminosity evolution (PLE), or whether
they formed more recently through the merging of spiral galaxies
(e.g. Baugh et al. 1996; Kauffmann 1996).
Studies on small fields claimed a deficit of EROs, thus favouring the
hierarchical merging scenario (e.g. Zepf 1997; Barger et al. 1999;
Rodighiero et al. 2001), but recent surveys on wider fields showed
that the surface density of EROs is consistent with elliptical galaxy
PLE expectations (D00; Daddi et al. 2000b). Since only a
few old galaxies have been spectroscopically identified (e.g.
Spinrad et al. 1997; Cohen et al. 1999), their fraction among EROs
remained still unconstrained.
On the other hand, EROs may also be high-z starbursts and AGNs strongly
reddened by dust extinction. Such a possibility was confirmed by
the identification of this kind of galaxies among EROs, but, again,
these results were limited to a handful of objects (e.g. Graham & Dey
1996; Cimatti et al. 1998; Gear et al. 2000; Pierre et al. 2001;
Smith et al. 2001; Afonso et al. 2001), thus leaving undetermined the
relative fractions of old and dusty galaxies in ERO samples.
Because of the stringent test of galaxy formation scenarios
that EROs can provide, it is therefore of prime importance
to determine the relative fractions among the two classes of galaxies.
In this letter,
we report on the first results of deep VLT optical spectroscopy
of a complete and sizeable sample of EROs.
A cosmology with H0=70 km s-1 Mpc-1,
and
is adopted.
Copyright ESO 2002