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 deg2 region of the ROSAT North Ecliptic Pole (NEP) Survey. Clustering is detected at the 4 σ level at comoving scales in the interval r = 5-60 h-1 Mpc. Fitting the data with a power law of slope γ = 1.8, we find a correlation length of r0 = 7.4 h-1 Mpc (ΩM = 0.3, ΩΛ = 0.7). The median redshift of the AGN contributing to the signal is zξ = 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 z ~ 2.2. 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, bX ~ 1. 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, fX = 1.1 × 10-13 ergs cm-2 s-1, and LX = 9.2 × 1043 h 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.