Issue |
A&A
Volume 409, Number 1, October I 2003
|
|
---|---|---|
Page(s) | 79 - 90 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20031031 | |
Published online | 17 November 2003 |
The HELLAS2XMM survey*,**
IV. Optical identifications and the evolution of the accretion luminosity in the Universe
1
INAF-Osservatorio Astronomico di Roma, via Frascati 33, Monteporzio-Catone (RM), 00040, Italy
2
Dip. di Astronomia Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
3
INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
4
Dip. di Fisica Università di Roma Tor Vergata, via della ricerca scientifica, 1, 00133 Roma, Italy
5
IASF/CNR, via Bassini 15, Milano, Italy
6
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
7
Dip. di Fisica, Università Roma Tre, via della vasca navale 84, 00146 Roma, Italy
8
INAF-Osservatorio Astrofisico di Arcetri, L. go E. Fermi 5, 50125 Firenze, Italy
9
INAF-Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
10
Dept. of Astronomy and Astrophysics Pennsylvania State University, 525 Davey, University Park, PA 16802, USA
Corresponding author: F. Fiore, fiore@mporzio.astro.it
Received:
22
April
2003
Accepted:
1
July
2003
We present results from the photometric and spectroscopic
identification of 122 X-ray sources recently discovered by XMM-Newton
in the 2–10 keV band (the HELLAS2XMM 1dF sample). Their flux cover the
range
and the total area
surveyed is 0.9 square degrees. One of the most interesting results
(which is found also in deeper sourveys) is that about 20% of the
hard X-ray selected sources have an X-ray to optical flux ratio (X/O)
ten times or more higher than that of optically selected AGN. Unlike
the faint sources found in the ultra-deep Chandra and XMM-Newton
surveys, which reach X-ray (and optical) fluxes more than one order of
magnitude lower than the HELLAS2XMM survey sources, many of the
extreme X/O sources in our sample have
and are therefore
accessible to optical spectroscopy. We report the identification of
13 sources with
(to be compared with 9 sources known from
the deeper, pencil-beam surveys). Eight of them are narrow line QSO
(seemingly the extension to very high luminosity of the type 2 Seyfert
galaxies), four are broad line QSO. The results from our survey are
also used to make reliable predictions about the luminosity of the
sources not yet spectroscopically identified, both in our sample and
in deeper Chandra and XMM-Newton samples. We then use a combined
sample of 317 hard X-ray selected sources (HELLAS2XMM 1dF, Chandra
Deep Field North 1Msec, Chandra SSA13 and XMM-Newton Lockman Hole flux
limited samples), 221 with measured redshifts, to evaluate the
cosmological evolution of the hard X-ray source's number and
luminosity densities. Looking backward in time, the low luminosity
sources (
erg s-1) increase in number at a
much slower rate than the very high luminosity sources
(
erg s-1), reaching a maximum around
and then levelling off beyond
. This
translates into an accretion driven luminosity density which is
dominated by sources with
erg s-1 up to at
least
, while the contribution of the same sources and of those
with
erg s-1 appear, with yet rather large
uncertainties, to be comparable between
and 4.
Key words: X-ray: diffuse background / X-ray: galaxies / quasars: general / surveys
© ESO, 2003
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