Spatial Correlation Function of X-Ray-selected Active Galactic Nuclei
Abstract
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=5-60h-1 Mpc. Fitting the data with a power law of slope γ=1.8, we find a correlation length of r0=7.4+1.8-1.9h-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=9.2×1043h-270 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 2004
- DOI:
- 10.1086/425248
- arXiv:
- arXiv:astro-ph/0408304
- Bibcode:
- 2004ApJ...617..192M
- Keywords:
-
- Cosmology: Observations;
- Cosmology: Large-Scale Structure of Universe;
- Galaxies: Quasars: General;
- X-Rays: General;
- Astrophysics
- E-Print:
- 17 pages, 8 figures, accepted for publication in ApJ, a version with high-resolution figures is available at http://www.eso.org/~cmullis/papers/Mullis_et_al_2004b.ps.gz, a machine-readable version of the ROSAT NEP AGN catalog is available at http://www.eso.org/~cmullis/research/nep-catalog.html