On the occupation of X-ray-selected galaxy groups by radio active galactic nuclei since z = 1.3

Previous clustering analysis of low-power radio active galactic nuclei (AGNs) has indicated that they preferentially live in massive groups. The X-ray surveys of the Cosmic Evolution Survey (COSMOS) field have achieved a sensitivity at which these groups are directly detected out to z = 1.3. Making use of the surveys Chandra, XMM-Newton and VLA-COSMOS, we identify radio AGN members [10^23.6≲ L_{1.4 GHz}/(W Hz^-1) ≲ 10²⁵] of galaxy groups (10^13.2≲ M₂₀₀/M_⊙≲ 10^14.4; 0.1 < z < 1.3) and study (i) the radio-AGN-X-ray group occupation statistics as a function of group mass; and (ii) the distribution of radio AGNs within the groups. We find that radio AGNs are preferentially associated with galaxies close to the centre (<0.2r₂₀₀). Compared to our control sample of group members matched in stellar mass and colour to the radio-AGN host galaxies, we find a significant enhancement of radio-AGN activity associated with 10^13.6≲ M₂₀₀/M_⊙≲ 10¹⁴ haloes. We present the first direct measurement of the halo occupation distribution (HOD) for radio AGNs, based on the total mass function of galaxy groups hosting radio AGNs. Our results suggest a possible deviation from the usually assumed power-law HOD model. We also find an overall increase in the fraction of radio AGNs in galaxy groups (<1r₂₀₀), relative to that in all environments. Based on observations with the National Radio Astronomy Observatory which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc., and with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.