2013MNRAS.428.1382K

We present the spatial clustering properties of 1466 X-ray-selected active galactic nuclei (AGN) compiled from the Chandra Deep Field (CDF) North and South, extended CDF-S, COSMOS and All-wavelength Extended Groth strip International Survey (AEGIS) fields in the 0.5-8 keV band. The X-ray sources span the redshift interval 0 < z < 3 and have a median value of {bar}z=0.976. We employ the projected two-point correlation function to infer the spatial clustering and find a clustering length of r0 = 7.2±0.6h-1 Mpc and a slope of γ = 1.48 ±0.12, which corresponds to a bias of b({bar}z)=2.26 ±0.16. Using two different halo bias models, we consistently estimate an average dark-matter host halo mass of Mh ≃ 1.3(± 0.3)x1013h-1M☉. The X-ray AGN bias and the corresponding dark-matter host halo mass are significantly higher than the corresponding values of optically selected AGN (at the same redshifts). The redshift evolution of the X-ray-selected AGN bias indicates, in agreement with other recent studies, that a unique dark-matter halo mass does not fit well the bias at all the different redshifts probed. Furthermore, we investigate if there is a dependence of the clustering strength on X-ray luminosity. To this end we consider only 650 sources around z ∼ 1 and we apply a procedure to disentangle the dependence of clustering on redshift. We find indications for a positive dependence of the clustering length on X-ray luminosity, in the sense that the more luminous sources have a larger clustering length and hence a higher dark-matter halo mass. In detail we find for an average luminosity difference of δ log10Lx ≃ 1 a halo mass difference of a factor of ∼ 3.

These findings appear to be consistent with a galaxy formation model where the gas accreted on to the supermassive black hole in intermediate-luminosity AGN comes mostly from the hot-halo atmosphere around the host galaxy.