  We present deep GMOS-S/Gemini optical broad-band images for a complete sample
of 46 southern 2Jy radio galaxies at intermediate redshifts (0.05<z<0.7). The
high-quality observations show for the first time that the overall majority of
PRGs at intermediate redshifts (78-85%) show peculiarities in their optical
morphologies at relatively high levels of surface brightness ($\mu$v=23.6 and
{\Delta}$\mu$v~[21,26] mag arcsec-2) including tails, fans, bridges, shells,
dust lanes, irregular features, amorphous haloes, and multiple nuclei. While
the results for many of the galaxies are consistent with them being observed
at, or after, the time of coalescence of the nuclei in a galaxy merger, we find
that more than 1/3 of the sample are observed in a pre-coalescence phase of the
merger, or following a close encounter between galaxies. By dividing the sample
into Weak-Line Radio Galaxies (WLRGs; 11 objects) and Strong-Line Radio
Galaxies (SLRGs; 35 objects) we find that only 27% of the former show clear
evidence for interactions in their optical morphologies, in contrast to the
SLRGs, of which at least 94% appear interacting. This is consistent with the
idea that many WLRGs are fuelled/triggered by Bondi accretion of hot gas. Of
the 28% of the sample that display evidence for significant starburst activity,
we find that 92% present disturbed morphologies, following the same general
trend as the total and SLRG samples. By comparing our PRGs with various samples
of quiescent ellipticals from the literature, we conclude that the percentage
of morphological disturbance that we find here exceeds that found for quiescent
ellipticals when similar surface brightnesses are considered. Overall, our
study indicates that galaxy interactions are likely to play a key role in the
triggering of AGN/jet activity.
