Occupation of X-ray-selected galaxy groups by X-ray active galactic nuclei

Abstract

We present the first direct measurement of the mean halo occupation distribution (HOD) of X-ray-selected active galactic nuclei (AGNs) in the COSMOS field at z ≤ 1, based on the association of 41 XMM and 17 C-COSMOS AGNs with member galaxies of 189 X-ray-detected galaxy groups from XMM-Newton and Chandra data. We model the mean AGN occupation in the halo mass range log M 200 [M⊙] = 13-14.5 with a rolling-off power law with the best-fit index α = 0.06(- 0.22; 0.36) and normalization parameter fa = 0.05(0.04; 0.06). We find the mean HOD of AGNs among central galaxies to be modeled by a softened step function at logMh >logMmin = 12.75(12.10, 12.95)M⊙ while for the satellite AGN HOD we find a preference for an increasing AGN fraction with Mh , suggesting that the average number of AGNs in satellite galaxies grows slower (αs < 0.6) than the linear proportion (αs = 1) observed for the satellite HOD of samples of galaxies. We present an estimate of the projected autocorrelation function (ACF) of galaxy groups over the range of rp = 0.1-40 h -1Mpc 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 AGNs that are within galaxy groups and we model the two-halo term of the clustering signal with the mean AGN HOD based on our results. © 2012. The American Astronomical Society. All rights reserved.