Issue |
A&A
Volume 424, Number 2, September III 2004
|
|
---|---|---|
Page(s) | 545 - 559 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20041153 | |
Published online | 23 August 2004 |
Discovery of optically faint obscured quasars with Virtual Observatory tools
1
ST-ECF, European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany e-mail: Paolo.Padovani@eso.org
2
Centre de Données astronomiques de Strasbourg (UMR 7550), 11 rue de l'Université, 67000 Strasbourg, France e-mail: allen@astro.u-strasbg.fr
3
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany e-mail: Piero.Rosati@eso.org
4
Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK e-mail: naw@ast.cam.ac.uk
Received:
23
April
2004
Accepted:
2
June
2004
We use Virtual Observatory (VO) tools to identify optically faint,
obscured (i.e., type 2) active galactic nuclei (AGN) in the two Great
Observatories Origins Deep Survey (GOODS) fields. By employing publicly
available X-ray and optical data and catalogues we discover 68 type 2 AGN
candidates. The X-ray powers of these sources are estimated by using a
previously known correlation between X-ray luminosity and X-ray-to-optical
flux ratio. Thirty-one of our candidates have high estimated powers
( erg/s) and therefore qualify as optically obscured quasars, the
so-called “QSO 2”. Based on the derived X-ray powers, our candidates are
likely to be at relatively high redshifts,
, with the QSO 2 at
. By going ~3 mag fainter than previously known type 2 AGN in the two GOODS fields we are sampling a region of redshift – power space
which was previously unreachable with classical methods. Our method brings to 40 the number of QSO 2 in the GOODS fields, an improvement of a factor
when compared to the only 9 such sources previously known. We derive
a QSO 2 surface density down to 10-15 erg cm-2 s-1 in the
keV band of
330 deg-2, ~
of which is made up of previously known sources. This
is larger than current estimates and some predictions and suggests that the surface
density of QSO 2 at faint flux limits has been underestimated. This work
demonstrates that VO tools are mature enough to produce cutting-edge science
results by exploiting astronomical data beyond “classical” identification
limits (
) with interoperable tools for statistical identification of
sources using multiwavelength information.
Key words: astronomical data bases: miscellaneous / methods: statistical / galaxies: quasars: general / X-rays: galaxies
© ESO, 2004
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