Clustering, Bias and the Accretion Mode of X-ray selected AGN

Abstract: We present the spatial clustering properties of 1466 X-ray selected AGN compiled from the Chandra CDF-N, CDF-S, eCDF-S, COSMOS and 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 Med{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+-0.6 h^{-1} Mpc and a slope of \gamma=1.48+-0.12, which corresponds to a bias of b=2.26+-0.16. Using two different halo bias models, we consistently estimate an average dark-matter host halo mass of Mh\sim 1.3 (+-0.3) x 10^{13} h^{-1} M_sun. 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). %indicating different populations of AGN. 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 \delta\log_{10} L_x ~ 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 onto the supermassive black hole in intermediate luminosity AGN comes mostly from the hot-halo atmosphere around the host galaxy.