Spatial correlation function of X-ray-selected active galactic nuclei

We present a detailed description of the first direct measurement of the spatial correlation function of X-ray selected active galactic nuclei (AGNs). This result is based on an X-ray flux-limited sample of 219 AGNs discovered in the contiguous 80.7 deg(2) region of the ROSAT North Ecliptic Pole (NEP) Survey. Clustering is detected at the 4 sigma level at comoving scales in the interval r=5-60 h(-1) Mpc. Fitting the data with a power law of slope gamma=1.8, we find a correlation length of r(0)=7.4(-1.9)(+1.8) h(-1) Mpc (Omega(M)=0.3, Omega(Lambda)=0.7). The median redshift of the AGN contributing to the signal is z(xi)=0.22. This clustering amplitude implies that X-ray-selected AGNs are spatially distributed in a manner similar to that of optically selected AGNs. Furthermore, the ROSAT NEP determination establishes the local behavior of AGN clustering, a regime that is poorly sampled in general. Combined with high-redshift measures from optical studies, the ROSAT NEP results argue that the AGN correlation strength essentially does not evolve with redshift, at least out to zsimilar to2.2. In the local universe, X-ray-selected AGNs appear to be unbiased relative to galaxies, and the inferred X-ray bias parameter is near unity, b(X)similar to1. Hence, X-ray-selected AGNs closely trace the underlying mass distribution. The ROSAT NEP AGN catalog, presented here, features complete optical identifications and spectroscopic redshifts. The median redshift, X-ray flux, and X-ray luminosity are z=0.41, f(X)=1.1x10(-13) ergs cm(-2) s(-1), and L-X=9.2x10(43) h(70)(-2) ergs s(-1) (0.5-2.0 keV), respectively. Unobscured, type I AGNs are the dominant constituents (90%) of this soft X-ray-selected sample of AGNs.