Clustering of X-ray selected active galactic nuclei

A total of 235 active galactic nuclei (AGN) from two different soft X-ray surveys [the ROSAT Deep Survey (DRS) and the ROSAT International X-ray Optical Survey (RIXOS)] with redshifts between 0 and 3.5 are used to study the clustering of X-ray selected AGN and its evolution. A 2 sigma significant detection of clustering of such objects is found on scales < 40-80 h(-1) Mpc in the RIXOS sample, while no clustering is detected on any scales in the DRS sample. Assuming a single power-law model for the spatial correlation function (SCF), quantitative limits on the AGN clustering have been obtained: a comoving correlation length 1.5 less than or similar to r(0) less than or similar to 3.3 h(-1) Mpc is implied for comoving evolution, while 1.9 less than or similar to r(0) less than or similar to 4.8 for stable clustering and 2.2 less than or similar to r(0) less than or similar to 5.5 for linear evolution. These values are consistent with the correlation lengths and evolutions obtained for galaxy samples, but imply smaller amplitude or faster evolution than recent ultraviolet and optically selected AGN samples. We also constrain the ratio of bias parameters between X-ray selected AGN and IRAS galaxies to be less than or similar to 1.7 on scales less than or similar to 10 h(-1) Mpc, a somewhat smaller value than is inferred from local large-scale dynamical studies.