The environments of luminous radio galaxies and type-2 quasars

We present the results of a comparison between the environments of (1) a complete sample of 46 southern 2-Jy radio galaxies at intermediate redshifts (0.05 <z< 0.7 in the Extended Groth Strip. The environments have been quantified using angular clustering amplitudes (Bgq) derived from deep optical imaging data. Based on these comparisons, we discuss the role of the environment in the triggering of powerful radio-loud and radio-quiet quasars. When we compare the Bgq distributions of the type-2 quasars and quiescent early-type galaxies, we find no significant difference between them. This is consistent with the radio-quiet quasar phase being a short-lived but ubiquitous stage in the formation of all massive early-type galaxies. On the other hand, powerful radio galaxies are in denser environments than the quiescent population, and this difference between distributions of Bgq is significant at the 3? level. This result supports a physical origin of radio loudness, with high-density gas environments favouring the transformation of active galactic nucleus (AGN) power into radio luminosity, or alternatively, affecting the properties of the supermassive black holes themselves. Finally, focusing on the radio-loud sources only, we find that the clustering of weak-line radio galaxies (WLRGs) is higher than the strong-line radio galaxies (SLRGs), constituting a 3? result. 82 per cent of the 2-Jy WLRGs are in clusters, according to our definition (Bgq 400), versus only 31 per cent of the SLRGs.ACKNOWLEDGEMENTS CRA acknowledges financial support from the Spanish Ministry of Science and Innovation (MICINN) through project ConsoliderIngenio 2010 Program grant CSD2006-00070: First Science with the GTC (http://www.iac.es/consolider-ingenio-gtc/); the Estallidos group through project PN AYA2010-21887-C04.04 and STFC PDRA (ST/G001758/1). PSB acknowledges support in the form of an STFC PhD studentship. KJI is supported through the Emmy Noether programme of the German Science Foundation (DFG). This research has made use of the NASA/IPAC Extragalactic Database (NED) and the NASA/IPAC Infrared Science Archive, which are operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. GAIA was created by the now closed Starlink UK project, funded by the Particle Physics and Astronomy Research Council (PPARC) and has been more recently supported by the Joint Astronomy Centre Hawaii funded again by PPARC and more recently by its successor organization the Science and Technology Facilities Council (STFC). We finally acknowledge thorough and useful comments from the anonymous referee