The dependence of triggering mechanisms on radio AGN subtypes: the role of galaxy mergers

ABSTRACT Powerful, radio-loud active galactic nuclei (AGN) are associated with one of the most important forms of AGN feedback, and understanding how they are triggered is key to properly incorporating them into models of galaxy evolution. Here, we present the results of a deep Isaac Newton Telescope/Wide Field Camera imaging survey which, when combined with Gemini/Gemini Multi-Object Spectrograph South images, gives a 98 per cent complete sample of 112 3CR radio galaxies with redshifts z < 0.3, alongside a stellar mass matched control sample. Our results provide strong evidence for significant differences (${\sim} 3\sigma$) between the triggering mechanisms of the different sub-types of powerful radio AGN. The high-excitation radio galaxies (HERGs) show a high rate of morphological disturbance (62$^{+6}_{-7}$ per cent) – an excess of ${\sim} 4\sigma$ compared with the control sample – consistent with them being predominantly triggered in galaxy mergers and interactions. In contrast, the low-excitation radio galaxies (LERGs) show a much lower rate of morphological disturbance (36$^{+7}_{-6}$ per cent), consistent with the control sample, and suggesting a different triggering mechanism, such as the accretion of gas from the hot X-ray haloes of the host galaxies or galaxy clusters. We also demonstrate that, when considering the radio morphology, the Fanaroff–Riley Class II (FRII) HERG sources preferentially reside in disturbed morphologies, a difference of ${\sim} 3\sigma$ to the FRII LERG objects. This suggests that the FRII LERG sources do not solely represent a ‘switched-off’ phase in the HERG lifecycle of the same parent galaxy population as the FRII HERGs.