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 2Jy radio galaxies at intermediate redshifts
(0.05 < z < 0.7), 2) a complete sample of 20 radio-quiet type-2 quasars (0.3 <
z < 0.41), and 3) a control sample of 107 quiescent early-type galaxies at 0.2
< z < 0.7 in the Extended Groth Strip (EGS). 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,
PRGs are in denser environments than the quiescent population, and this
difference between distributions of Bgq is significant at the 3 sigma level.
This result supports a physical origin of radio loudness, with high density gas
environments favouring the transformation of AGN power into radio luminosity,
or alternatively, affecting the properties of the supermassive black holes
themselves. Finally, focussing 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 sigma result. 82% of the 2Jy WLRGs are
in clusters, according to our definition (Bgq > 400) versus only 31% of the
SLRGs.
