The soft X-ray cluster-AGN spatial cross-correlation function in the ROSAT-NEP survey

¹ Max Planck Institute für Extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany e-mail: cappelluti@mpe.mpg.de
² California Institute of Technology, Mail Code 130-33, Pasadena, CA 91125, USA
³ Physics and Astronomy Department, University of Southern California, Los Angeles, CA 90089-0484, USA
⁴ Department of Astronomy, University of Michigan, 918 Dennison, 500 Church Street, Ann Arbor, 48109-1042, USA
⁵ Istituto di Radioastronomia INAF, via P. Gobetti 101, 40129 Bologna, Italy
⁶ Institute for Astronomy, University of Hawai'i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

Received: 27 June 2006
Accepted: 16 November 2006

Abstract

Context.X-ray surveys facilitate investigations of the environment of AGNs. Deep Chandra observations have revealed that the AGN source surface density rises near clusters of galaxies. The natural extension of this finding is the measurement of spatial clustering of AGNs around clusters and the investigation of relative biasing between active galactic nuclei and galaxies near clusters.

Aims.We aim to measure the correlation length of AGNs around clusters and the average clustering properties of a complete sample of AGNs in a dense environment.

Methods.We present the first measurement of the soft X-ray cluster-AGN cross-correlation function in redshift space using the data of the ROSAT-NEP survey. The survey covers 9  9 deg² around the North Ecliptic Pole where 442 X-ray sources were detected and almost completely spectroscopically identified.

Results.We detected a >3σ significant clustering signal on scales s ≤ 50  Mpc. We performed a classical maximum-likelihood power-law fit to the data and obtained a correlation length s₀ = 8.7  Mpc and a slope γ = 1.7 (1σ errors).

Conclusions.This is strong evidence that AGNs are good tracers of the large scale structure of the Universe. Our data were compared to the results obtained by cross-correlating X-ray clusters and galaxies. We observe, with a large uncertainty, a similar behaviour of AGN clustering around clusters similar to the clustering of galaxies around clusters.