Mergers are the dominant trigger for high luminosity HERGs
Considerable uncertainty remains about the triggering of radio AGN. Here we present the results of deep imaging observations of a large sample of high-excitation radio galaxies (HERGs) covering a wide range of radio power (10^22.5 < L_1.4 < 10^28 W/Hz) and [OIII] emission line luminosity (10^32.5 < L_[OIII]) < 10^36.5 W). We find that the rate of detection of tidal features suggestive of galaxy mergers increases with [OIII] emission-line luminosity. At the highest, quasar-like [OIII] luminosities (L_[OIII] > 10^35 W), the HERGs show a rate of morphological disturbance that is 3x higher than that of a sample of non-AGN control galaxies matched in stellar mass and redshift – similar to recent results for samples of nearby radio-quiet quasars. In contrast, at the lowest [OIII] luminosities the rate of morphological disturbance in the HERGs is consistent with that of the controls. Moreover, the overall morphologies of the host galaxies become increasing dominated by late-type morphologies at low luminosities. Overall, these results are consistent with galaxy mergers dominating the triggering of HERGs at high {OIII] luminosities, and secular processes becoming increasingly important at low luminosities.