Are luminous radio-loud active galactic nuclei triggered by galaxy
  interactions?
  We present the results of a comparison between the optical morphologies of a
complete sample of 46 southern 2Jy radio galaxies at intermediate redshifts
(0.05<z<0.7) and those of two control samples of quiescent early-type galaxies:
55 ellipticals at redshifts z<0.01 from the Observations of Bright Ellipticals
at Yale (OBEY) survey, and 107 early-type galaxies at redshifts 0.2<z<0.7 in
the Extended Groth Strip (EGS). Based on these comparisons, we discuss the role
of galaxy interactions in the triggering of powerful radio galaxies (PRGs). We
find that a significant fraction of quiescent ellipticals at low and
intermediate redshifts show evidence for disturbed morphologies at relatively
high surface brightness levels, which are likely the result of past or on-going
galaxy interactions. However, the morphological features detected in the galaxy
hosts of the PRGs (e.g. tidal tails, shells, bridges, etc.) are up to 2
magnitudes brighter than those present in their quiescent counterparts. Indeed,
if we consider the same surface brightness limits, the fraction of disturbed
morphologies is considerably smaller in the quiescent population (53% at z<0.2
and 48% at 0.2<z<0.7) than in the PRGs (93% at z<0.2 and 95% at 0.2<z<0.7
considering strong-line radio galaxies only). This supports a scenario in which
PRGs represent a fleeting active phase of a subset of the elliptical galaxies
that have recently undergone mergers/interactions. However, we demonstrate that
only a small proportion (<20%) of disturbed early-type galaxies are capable of
hosting powerful radio sources.
