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

Abstract

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 × 10^43 erg s^−1, respectively. A strong clustering signal is detected at ∼18σ 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 γ = 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 ∼ 5−15 h^−1 Mpc, which is due to a large-scale structure at z ∼ 0.36 containing about 40 AGN, a feature which is evident over many wavelengths in the COSMOS field. When removing the z ∼ 0.36 structure or computing w(rp) 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 ∼ 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 ∼ 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 ∼ 1 is similar to that of massive galaxies (stellar mass M* ≳ 3 × 10^10 M⊙) at the same redshift. This suggests that AGN at z ∼ 1 are preferentially hosted by massive galaxies, as observed both in the local and in the distant (z ∼ 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 ∼ 8 h^−1 Mpc by z = 0, and hence to be hosted by local bright (L ∼ L*) ellipticals. We make use of dark matter halo catalogs from the Millennium simulation to determine the typical halo hosting moderately luminous z ∼ 1 AGN. We find that XMM-COSMOS AGN live in halos with masses M ≳ 2.5 × 10^12 M⊙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 ∼ 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.

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