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

Abstract

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.