X-ray selected AGN in groups at redshifts z ≈ 1

Abstract

We explore the role of the group environment in the evolution of active galactic nuclei (AGN) at the redshift interval 0.7 < z < 1.4, by combining deep Chandra observations with extensive optical spectroscopy from the All-wavelength Extended Groth strip International Survey (AEGIS). The sample consists of 3902 optical sources and 71 X-ray AGN. Compared to the overall optically selected galaxy population, X-ray AGN are more frequently found in groups at the 99 per cent confidence level. This is partly because AGN are hosted by red luminous galaxies, which are known to reside, on average, in dense environments. Relative to these sources, the excess of X-ray AGN in groups is significant at the 91 per cent level only. Restricting the sample to 0.7 < z < 0.9 and MB < -20 mag in order to control systematics, we find that X-ray AGN represent (4.7 ± 1.6) and (4.5 ± 1.0) per cent of the optical galaxy population in groups and in the field, respectively. These numbers are consistent with the AGN fraction in low-redshift clusters, groups and the field. The above results, although affected by small number statistics, suggest that X-ray AGN are spread over a range of environments, from groups to the field, once the properties of their hosts (e.g. colour, luminosity) are accounted for. There is also tentative evidence, significant at the 98 per cent level, that the field produces more X-ray luminous AGN compared to groups, extending similar results at low redshift to z ≈ 1. This trend may be because of either cold gas availability or the nature of the interactions occurring in the denser group environment (i.e. prolonged tidal encounters). © 2008 RAS.