Issue |
A&A
Volume 466, Number 1, April IV 2007
|
|
---|---|---|
Page(s) | 31 - 40 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20065170 | |
Published online | 22 January 2007 |
The HELLAS2XMM survey*,**
VIII. Optical identifications of the extended sample
1
INAF – Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio-Catone (RM) 00040, Italy e-mail: cocchia@brera.mi.astro.it
2
Dipartimento di Fisica, Università di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy
3
INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
4
Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
5
INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
6
Max Planck Institüt für Extraterrestrische Physik (MPE), Giessenbachstr. 1, 85748 Garching, Germany
7
Harvard-Smithsonian Center for Astrophysics (CfA), 60 Garden str, Cambridge 02138 MA, USA
8
INAF – IASF, via Bassini 15, 20133 Milano, Italy
9
Dipartimento di Fisica, Università Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
10
INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
11
ASI Science Data Center, ASDC c/o ESRIN, via G. Galilei, 00044 Frascati, Italy
Received:
9
March
2006
Accepted:
1
December
2006
Aims.Hard X-ray, large-area surveys are a fundamental complement to ultra-deep, pencil-beam surveys in obtaining more complete coverage of the AGN luminosity-redshift plane and finding sizeable samples of “rare” AGN.
Methods.We present the results of the photometric and spectroscopic
identification of 110 hard X-ray selected sources from 5 additional
XMM-Newton fields, nearly doubling the original HELLAS2XMM
sample. Their 2–10 keV fluxes cover the range
6
10-13
and the total area
surveyed is ~0.5 deg2 at the bright flux limit. We
spectroscopically identified 59 new sources, bringing the
spectroscopic completeness of the full HELLAS2XMM sample to almost
70% over a total area of ~1.4 deg2 at the bright flux
limit. We found optical counterparts for 214 out of the 232 X-ray
sources of the full sample down to
. We measured the flux
and luminosity of the [OIII]
emission line for 59 of these sources.
Results.Assuming that most high X-ray-to-optical flux ratio sources are
obscured QSOs, we used the full HELLAS2XMM sample and the CDF samples to estimate their . We find obscured QSOs surface density of 45 ± 15 and 100–350 deg-2 down to flux limits of 10-14 and 10-15
, respectively. At these flux
limits, the fraction of X-ray-selected obscured QSOs turns out to be
similar to that of unobscured QSOs. Since X-ray selection misses most
Compton-thick AGN, the number of obscured QSOs may well outnumber the unobscured QSOs.
We find that hard X-ray selected AGNs with a detected [OIII] emission
span a wide range of
with a logarithmic
median of 2.14 and interquartile range of 0.38. This is marginally higher than for a sample of
optically selected AGNs (median 1.69 and
interquatile range 0.30), suggesting that optically
selected samples are at least partly incomplete and
or that [OIII] emission
is not a perfect isotropic indicator of the nuclear power.
The seven X-ray bright, optically normal galaxy (XBONG) candidates in
the sample have
, while their X-ray
and optical luminosities and obscuring column density are similar to
those of narrow-line AGNs in the same redshift interval (0.075–0.32). This suggests that, while the central engine of narrow-line AGNs and XBONGs looks similar, the narrow-line region in
XBONGs could be strongly inhibited or obscured.
Key words: X-rays: diffuse background / surveys / galaxies: active / galaxies: evolution
© ESO, 2007
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