Clustering, bias and the accretion mode of X-ray-selected AGN

We present the spatial clustering properties of 1466 X-ray s elected AGN compiled from the ChandraCDF-N, CDF-S, eCDF-S, COSMOS and AEGIS fields in the0.5−8 keV band. The X-ray sources span the redshift interval 0 < z < 3 and have a median value of ¯ = 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 = 1.48 ± 0.12, which corresponds to a bias of b(¯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) × 10 13 h −1 M⊙. The Xray 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 c lustering 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 da rk-matter halo mass. In detail we