The environment of active galactic nuclei : excess counts and the nature of their host clusters

Sammanfattning: This thesis presents observations of the galaxy environment of active galactic nuclei on galaxy cluster scales. In the first part, we quantify the galaxy environment of a sample of 40 quasars, consisting of equal fractions of radio-loud and radio-quiet sources. The sample covers a narrow range in redshift, 0.5 < z < 0.8, but a wide range in quasar luminosity so that the luminosity-redshift degeneracy is eliminated. This allows us to investigate how environmental richness depends on luminosity, and we found that the environments of radio-loud quasars display a weak correlation with radio luminosity. There was no evidence for a redshift evolution in the environmental richness as has been previously claimed, and we suggest that selection effects, caused by the underlying relation between environmental density and radio luminosity, might cause an apparent evolution with redshift. We found that the quasars exist in a wide variety of environments, from apparently rich clusters to field-like environments. The average galaxy environment was found to be approximately three times richer than that of field galaxies in the local Universe, corresponding to groups of galaxies or poorer clusters. Contrary to many previous studies, the average environments of radio-loud and radio-quiet quasars were found to be statistically indistinguishable, implying that radio-loud and radio-quiet quasars exist in similar environments on 0.5 Mpc scales.In the second part of the thesis, we present weak lensing observations of four deep fields centered on powerful active galactic nuclei. Two previously known clusters are comfortably detected, and we also report the detection of a previously unknown cluster-sized mass concentration ((2-4) x 10^14 h_{100}^{-1} solar masses in the field of the z=0.734 quasar 3C 254. The weak lensing result for one of the fields is still inconclusive with regard to the detection of a cluster. The weak lensing analysis allows us to constrain physical parameters of the clusters, and we discuss the results in light of the cooling flow and the merger/interaction scenarios for triggering and fuelling active galactic nuclei in clusters. Each scenario makes a different prediction about the dynamical state of the clusters; if mergers or interactions are important, the clusters should be dynamically young, but if cooling flows are important, the clusters are expected to be dynamically old and evolved systems. The dynamics of the clusters studied here do not unambiguously support either of the two scenarios. Whereas the clusters are massive and appear to have smooth mass distributions, typical for evolved clusters, one of the clusters shows evidence for past merger activity. We suggest that cluster mergers may be responsible for generating close encounters between, or even mergers of, the massive host galaxy and a cluster member. This could trigger the active galactic nucleus, and if the cluster member is a gas-rich galaxy, it might provide the fuel for the active galactic nucleus necessary to maintain its high luminosity.