Issue |
A&A
Volume 470, Number 1, July IV 2007
|
|
---|---|---|
Page(s) | 21 - 37 | |
Section | Cosmology (including clusters of galaxies) | |
DOI | https://doi.org/10.1051/0004-6361:20066497 | |
Published online | 24 April 2007 |
Unveiling the oldest and most massive galaxies at very high redshift*
1
Dipartimento di Astronomia, Università di Padova, Vicolo Osservatorio 2, 35122 Padova, Italy e-mail: giulia.rodighiero@unipd.it
2
Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
3
Max Planck Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching bei München, Germany
4
INAF – Bologna, via Ranzani, 40127 Bologna, Italy
Received:
3
October
2006
Accepted:
5
March
2007
Context. The identification and characterisation of massive galaxies over a wide redshift range allow us to place stringent constraints on the cosmic history of galaxy mass assembly and on current models of galaxy formation and evolution.
Aims. This work explores the existence of high-redshift massive galaxies unveiled with Spitzer+IRAC, but missed by conventional selection techniques based on optical and near-infrared observations.
Methods.
To this end, we
used multi-wavelength imaging data available for the GOODS-South
field (130 arcmin2) and selected a flux-limited sample from the
IRAC 3.6 μm image to Jy (m(AB) < 23.26).
In order to identify the most extreme objects and to complement previously
published selections
in this field, we confined our study to the galaxies undetected by the
optical HST+ACS imaging and close to the detection limit of the
K-band image (
AB). Our selection unveiled 20 galaxies
on which we performed a detailed analysis.
For each galaxy, we built a spectral energy distribution (SED) based on
optical-to-8μm photometry. The SEDs were then used to estimate the
photometric redshifts and to derive the main galaxies' physical
properties.
Further constraints were also obtained from the available X-ray and 24 μm data.
Results.
The majority of the sample (14 out of 20) sources show degenerate/bimodal solutions for
the photometric redshifts. These can either be heavily dust-enshrouded
() starbursts at
with bolometric luminosities
, or massive post-starburst
galaxies in the redshift interval
with stellar masses of ~
.
The remaining six galaxies present a less ambiguous
photometric redshift: with the exception of one low-z
dusty source, these latter objects favour a low-extinction solution, with
four of them showing best-fit photo-z solutions at
. One galaxy, ID-6, the only
source in
our sample with both an X-ray and a 24 μm detection, might be an extremely
massive object at
detected during a post-starburst phase with concomitant
QSO activity responsible for the 24 μm and X-ray emissions
(although a lower-z solution is not excluded).
Conclusions.
Our investigation of Spitzer-selected galaxies that have very red
SEDs and completely undetected in the optical reveals a potential
population of massive galaxies at , which appear to include significant
AGN emissions.
These sources may be the oldest stellar systems at
, given that the estimated ages are close to the age of the Universe at that redshift.
We found that these, previously unrecognised, optically obscured objects
might provide an important contribution to the massive end
(
) of the high-z stellar mass function, and they
would almost double it.
Our evidence in these mature high-z galaxies of the
widespread presence of hidden AGNs may have important implications for
galaxy formation, due to their feedback effects on the surrounding ISM.
Key words: galaxies: high-redshift / galaxies: formation / cosmology: observations / infrared: galaxies
© ESO, 2007
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