Occupation of X-ray selected galaxy groups by X-ray AGN
  We present the first direct measurement of the mean Halo Occupation
Distribution (HOD) of X-ray selected AGN in the COSMOS field at z < 1, based on
the association of 41 XMM and 17 C-COSMOS AGN with member galaxies of 189 X-ray
detected galaxy groups from XMM and Chandra data. We model the mean AGN
occupation in the halo mass range logM_200[Msun] = 13-14.5 with a rolling-off
power-law with the best fit index alpha = 0.06(-0.22;0.36) and normalization
parameter f_a = 0.05(0.04;0.06). We find the mean HOD of AGN among central
galaxies to be modelled by a softened step function at logMh > logMmin = 12.75
(12.10,12.95) Msun while for the satellite AGN HOD we find a preference for an
increasing AGN fraction with Mh suggesting that the average number of AGN in
satellite galaxies grows slower (alpha_s < 0.6) than the linear proportion
(alpha_s = 1) observed for the satellite HOD of samples of galaxies. We present
an estimate of the projected auto correlation function (ACF) of galaxy groups
over the range of r_p = 0.1-40 Mpc/h at <z> = 0.5. We use the large-scale
clustering signal to verify the agreement between the group bias estimated by
using the observed galaxy groups ACF and the value derived from the group mass
estimates. We perform a measurement of the projected AGN-galaxy group
cross-correlation function, excluding from the analysis AGN that are within
galaxy groups and we model the 2-halo term of the clustering signal with the
mean AGN HOD based on our results.
