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