Do AGN triggering mechanisms vary with radio power? I.Optical
  morphologies of radio-intermediate HERGs
  Radio AGNs with intermediate radio powers are capable of driving multi-phase
outflows in galaxy bulges, and are also more common than their high-radio-power
counterparts. In-depth characterisation of the typical host galaxies and likely
triggering mechanisms for this population is therefore required in order to
better understand the role of radio AGN feedback in galaxy evolution. Here, we
use deep optical imaging data to study the detailed host morphologies of a
complete sample of 30 local radio AGNs with high-excitation optical emission
(HERG) spectra and intermediate radio powers (z $<$ 0.1; 22.5 $<$ log(L$_{\rm
1.4GHz}$) $<$ 24.0 WHz$^{-1}$). The fraction of hosts with morphological
signatures of mergers and interactions is greatly reduced compared to the 2 Jy
radio-powerful galaxies (log(L$_{\rm 1.4GHz}$) $>$ 25.0 WHz$^{-1}$) with strong
optical emission lines: 53 $\pm$ 9 per cent and 94 $\pm$ 4 per cent,
respectively. In addition, the most radio-powerful half of the sample has a
higher frequency of morphological disturbance than the least radio-powerful
half (67 $\pm$ 12 per cent and 40 $\pm$ 13 per cent, respectively), including
the eight most highly-disturbed galaxies. This suggests that the importance of
triggering nuclear activity in HERGs through mergers and interactions reduces
with radio power. Both visual inspection and detailed light profile modelling
reveal a mixed population of early-type and late-type morphologies, contrary to
the massive elliptical galaxy hosts of radio-powerful AGNs. The prevalence of
late-type hosts could suggest that triggering via secular, disk-based processes
has increased importance for HERGs with lower radio powers (e.g. disk
instabilities, large scale bars).
