The spatial clustering of X-ray selected AGN in the XMM-COSMOS field

We study the spatial clustering of 538 X-ray selected AGN in the 2 deg(2) XMM-COSMOS field that are spectroscopically identified with I(AB) < 23 and span the redshift range z = 0.2-3.0. The median redshift and X-ray luminosity of the sample are z = 0.98 and L(0.5-10) = 6.3 x 10(43) erg s(-1), respectively. A strong clustering signal is detected at similar to 18 sigma level, which is the most significant measurement obtained to date for clustering of X-ray selected AGN. By fitting the projected correlation function w(r(p)) with a power law on scales of r(p) = 0.3-40 h(-1) Mpc, we derive a best-fit comoving correlation length of r(0) = 8.6 +/- 0.5 h(-1) Mpc and slope of. = 1.88 +/- 0.07 (Poissonian errors; bootstrap errors are about a factor of 2 larger). An excess signal is observed in the range r(p) similar to 5-15 h(-1) Mpc, which is due to a large-scale structure at z similar to 0.36 containing about 40 AGN, a feature which is evident over many wavelengths in the COSMOS field. When removing the z similar to 0.36 structure or computing w(r(p)) in a narrower range around the peak of the redshift distribution (e. g. z = 0.4-1.6), the correlation length decreases to r(0) similar to 5-6 h(-1) 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 z similar to 1. 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 z = 1. The correlation length measured for XMM-COSMOS AGN at z similar to 1 is similar to that of massive galaxies (stellar mass M(*) greater than or similar to 3 x 10(10) M(circle dot)) at the same redshift. This suggests that AGN at z similar to 1 are preferentially hosted by massive galaxies, as observed both in the local and in the distant (z similar to 2) 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 r(0) similar to 8 h(-1) Mpc by z = 0, and hence to be hosted by local bright (L similar to L(*)) ellipticals. We make use of dark matter halo catalogs from the Millennium simulation to determine the typical halo hosting moderately luminous z similar to 1 AGN. We find that XMM-COSMOS AGN live in halos with masses M greater than or similar to 2.5 x 10(12) M(circle dot) h(-1). 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 z similar to 1, 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.