Chandra point-source counts in distant galaxy clusters^*

¹ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: m.branchesi@ira.inaf.it
² Istituto di Radioastronomia, INAF, via Gobetti 101, 40129 Bologna, Italy e-mail: gioia@ira.inaf.it
³ Dipartimento di Fisica, Università di Bologna, via Irnerio 46, 40126 Bologna, Italy e-mail: [rfanti;cfanti]@ira.inaf.it
⁴ Max Planck Institute für Extraterrestrische Physik, Postfach 1312, 85741, Garching, Germany e-mail: cappelluti@mpe.mpg.de

Received: 6 August 2006
Accepted: 24 September 2006

Abstract

Context.With the superb angular resolution of the Chandra Observatory, it is now possible to detect X-ray point sources, either embedded in galaxy clusters or along the cluster line of sight, which could not be resolved by previous instruments. This now allows studies of source counts in distant cluster fields.

Aims.We want to analyze the inner region of clusters of galaxies to check for the presence of any overdensity of X-ray point sources embedded in the gas diffuse emission. These point sources are possible AGN belonging to the clusters and could contaminate the cluster emission.

Methods.We used a sample of 18 distant () galaxy clusters from the Chandra archive to construct the , in both the soft and hard energy bands, for the X-ray point sources detected in the central cluster region to be compared with the counts of point sources detected in similarly deep fields without clusters.

Results.We find a ~2σ excess of cluster region sources at the bright end of the . The radial distribution of the brightest X-ray point sources confirms this excess and indicates that it is confined to the inner 0.5 Mpc of the cluster region.

Conclusions.The results suggest the possible existence of X-ray sources belonging to the cluster (most probably AGN, given their 0.5-10 keV luminosity ranging from 10⁴³ to 10⁴⁴ erg s^-1): on average one every three clusters. Unlike previous studies, which have mainly investigated the point-source population in the vicinity of the galaxy clusters, the present study analyzes the content of point sources within the 1 Mpc region covered by the cluster extent. Our work confirms the findings of other investigators who analyzed the central 1 Mpc region of more massive clusters and/or groups in a similar redshift range. The X-ray source excess found here is much smaller than the excess of radio galaxies found recently in high-z X-ray selected clusters, possibly due to the better sensitivity of the radio observations.