High-precision multi-band measurements of the angular clustering of X-ray sources

J. Ebrero; S. Mateos; G. C. Stewart; F. J. Carrera; M. G. Watson

doi:doi:10.1051/0004-6361/200911670

Free access

Issue		A&A Volume 500, Number 2, June III 2009


Page(s)		749 - 762
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361/200911670
Published online		29 April 2009

A&A 500, 749-762 (2009)
DOI: 10.1051/0004-6361/200911670

High-precision multi-band measurements of the angular clustering of X-ray sources

J. Ebrero^{1, 2}, S. Mateos¹, G. C. Stewart¹, F. J. Carrera², and M. G. Watson¹

¹ Department of Physics and Astronomy, University of Leicester, University Road, LE1 7RH, Leicester, UK
e-mail: jec33@star.le.ac.uk
² Instituto de Física de Cantabria (CSIC-UC), Avenida de los Castros, 39005 Santander, Spain
e-mail: ecarrero@ifca.unican.es

Received 16 January 2009 / Accepted 6 April 2009

Abstract
Aims. We aim to study the large-scale structure of an extragalactic serendipitous X-ray survey with unprecedented accuracy thanks to the large statistics involved, and provide insight into the environment of AGN at the epochs when their space density declines ( $z \sim 1{-}2$ ).
Methods. In this paper we present the two-point angular correlation function of the X-ray source population of 1063 XMM-Newton observations at high Galactic latitudes, comprising up to ~30 000 sources over a sky area of ~125.5 deg², in three energy bands: 0.5–2 (soft), 2–10 (hard), and 4.5–10 (ultrahard) keV. This is the largest survey of serendipitous X-ray sources ever used for clustering analysis.
Results. We have measured the angular clustering of our survey and find significant positive clustering signals in the soft and hard bands (~10 $\sigma$ and ~5 $\sigma$ , respectively), and a marginal clustering detection in the ultrahard band (<1 $\sigma$ ). We find dependency of the clustering strength on the flux limit and no significant differences in the clustering properties between sources with high hardness ratios (and therefore likely to be obscured AGN) and those with low hardness ratios. We deprojected the angular clustering parameters via Limber's equation to compute their typical spatial lengths. From that we have inferred that AGN at redshifts of ~1 are embedded in dark matter haloes with typical masses of $\langle \log M_{\rm DMH}\rangle \simeq 12.60\pm0.34$ h^-1 $M_\odot$ and lifetimes of $t_{\rm AGN}=3.1{-}4.5 \times 10^8$ yr.
Conclusions. Our results show that obscured and unobscured objects share similar clustering properties and therefore they both reside in similar environments, in agreement with the unified model of AGN. The short AGN lifetimes derived suggest that AGN activity might be a transient phase that can be experienced several times by a large fraction of galaxies throughout their lives.

Key words: surveys -- X-rays: general -- cosmology: large-scale structure of Universe -- galaxies: active

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