Issue |
A&A
Volume 494, Number 1, January IV 2009
|
|
---|---|---|
Page(s) | 33 - 48 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:200810821 | |
Published online | 04 December 2008 |
The spatial clustering of X-ray selected AGN in the XMM-COSMOS field
1
INAF – Osservatorio Astronomico di Bologna, Via Ranzani 1, 40127 Bologna, Italy e-mail: roberto.gilli@oabo.inaf.it
2
Instituto de Astronomía, Universidad Nacional Autónoma de México, Ensenada, México (mailing address: PO Box 439027, San Ysidro, CA, 92143-9027, USA)
3
Institute of Astronomy, Swiss Federal Institute of Technology (ETH Hönggerberg), 8093, Zürich, Switzerland
4
Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
5
European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching 85748, Germany
6
Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, DAPNIA/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France
7
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
8
INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone (Roma), Italy
9
California Institute of Technology, MC 105-24, 1200 East California Boulevard, Pasadena, CA 91125, USA
10
Dipartimento di Astronomia, Università degli Studi di Bologna, Via Ranzani 1, 40127 Bologna, Italy
11
Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
12
Institut d'Astrophysique de Paris, 98 bis Boulevard Arago, 75014 Paris, France
13
Research Center for Space and Cosmic Evolution, Ehime University, Bunkyo-cho 2-5, Matsuyama 790-8577, Japan
14
Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue E. Belin, 31400 Toulouse, France
15
Laboratoire d'Astrophysique de Marseille, CNRS-Université de Provence, Traverse du Siphon, BP 8, 13012 Marseille, France
16
Dipartimento di Astronomia, Università di Padova, Vicolo Osservatorio 2, 35122 Padova, Italy
17
INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Bassini 15, 20133 Milan, Italy
18
INAF – Osservatorio Astronomico di Brera, 23807 Merate (LC), Italy
19
University of California, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94720, USA
20
Centre de Physique Théorique, UMR 6207 CNRS-Université de Provence, Case 907, 13288 Marseille, France
Received:
18
August
2008
Accepted:
22
October
2008
We study the spatial clustering of 538 X-ray selected AGN in the 2 deg2 XMM-COSMOS field that are spectroscopically identified with
and span the redshift range
. The median
redshift and X-ray luminosity of the sample are
and
erg s-1, respectively. A strong
clustering signal is detected at ~
level, which is the
most significant measurement obtained to date for clustering of X-ray
selected AGN. By fitting the projected correlation function
with a power law on scales of
Mpc, we derive a
best-fit comoving correlation length of
Mpc
and slope of
(Poissonian errors; bootstrap errors
are about a factor of 2 larger). An excess signal is observed in the
range
Mpc, which is due to a large-scale
structure at
containing about 40 AGN, a feature which is
evident over many wavelengths in the COSMOS field. When removing the
structure or computing
in a narrower range
around the peak of the redshift distribution (e.g.
), the
correlation length decreases to
Mpc, which is
consistent with what is observed for bright optical QSOs at the same
redshift.
We investigate the clustering properties of obscured and unobscured
AGN separately, adopting different definitions for the source
obscuration. For the first time, we are able to provide a significant
measurement for the spatial clustering of obscured AGN at
.
Within the statistical uncertainties, we do not find evidence that AGN
with broad optical lines (BLAGN) cluster differently from AGN without
broad optical lines (non-BLAGN).
Based on these results, which are limited by object statistics,
however, obscured and unobscured AGN are consistent with inhabiting
similar environments.
The evolution of AGN clustering with redshift is also investigated. No
significant difference is found between the clustering properties of
XMM-COSMOS AGN at redshifts below or above
.
The correlation length measured for XMM-COSMOS AGN at
is
similar to that of massive galaxies (stellar mass
) at the same redshift. This suggests
that AGN at
are preferentially hosted by massive galaxies,
as observed both in the local and in the distant (
)
Universe. According to a simple clustering evolution scenario, we find
that the relics of AGN are expected to have a correlation length as
large as
Mpc by
, and hence to be hosted by
local bright (
) ellipticals.
We make use of dark matter halo catalogs from the Millennium
simulation to determine the typical halo hosting moderately luminous
AGN. We find that XMM-COSMOS AGN live in halos with masses
. By combining the
number density of XMM-COSMOS AGN to that of the hosting dark matter
halos we estimate the AGN duty cycle and lifetimes. We find lifetimes
approximately of 1 Gyr for AGN at
, which are longer than
those estimated for optically bright QSOs at the same redshift. These
longer lifetimes mainly reflect the higher number density of AGN
selected by X-ray samples.
Key words: galaxies: active / cosmology: large-scale structure of Universe / cosmology: observations / X-rays: galaxies
© ESO, 2009
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