Detection of the first X-ray selected large AGN group

Abstract

We have examined the spatial distribution of 856 AGN detected by the ROSAT All-Sky Survey (RASS) using a direct search for structures with the minimal spanning tree. The AGNs were compiled from an area of ∼7000 deg2, in which optical identifications of RASS sources were made with the help of the digitized objective prism plates of the Hamburg Quasar Survey (HQS). Redshifts were taken from the literature or from own follow-up observations. The sample probes the spatial distribution at low redshifts, because the redshift distribution peaks at z∼0.1. The application of the minimal spanning tree led to a 1.8σ discovery of an AGN group with seven members in a volume V∼140×75×75 h−3 Mpc3 in the Pisces constellation. With a mean redshift z=0.27 this group is only the third discovered group at redshifts z<0.5. The RASS offers excellent possibilities to study large-scale structure with AGNs at low redshifts, once these redshifts are determined.