The optical morphologies and radio properties of HERGs with intermediate radio powers

Sensitive radio surveys with broad sky coverage have revealed that the local radio AGN population is dominated by sources with lower powers than those previously studied as representative of the class. High-excitation radio galaxies (HERGs) with intermediate radio powers (10^22.5 < L_1.4GHz < 10^25 W Hz^−1) represent a particularly important subset of these lower-power objects, given their strong association with kinematic disturbances in the multiphase gas of their host galaxies. Despite this, little is currently known about the properties of these radio-intermediate HERG sources. For this thesis, deep optical imaging observations of radio-intermediate HERGs have been used to characterise their detailed morphologies for the first time. The host morphologies of powerful radio AGN and Type 2 quasars have also been studied, as well those of stellar-mass- and redshift-matched non-active control galaxies. The rate of morphological disturbance for lower-power radio-intermediate HERGs is found to be relatively low and indistinguishable from the control sample. However, when the full active galaxy sample is considered, there is clear evidence to suggest that the enhancement in the rate of disturbance relative to the controls increases strongly with the AGN power, such that objects with quasar-like [OIII]λ5007 emission line luminosities exhibit a 5σ excess relative to matched non-active galaxies. This strongly supports the idea that merger-based triggering is dominant for the most luminous AGN. Evidence for a transition in the dominant AGN host types from early-type galaxies at high radio powers to late-type galaxies at low radio powers is also found, which may indicate that triggering by secular, disk-based processes holds more importance for radio AGN with more moderate luminosities. In addition, high-resolution radio observations of 16 radio-intermediate HERGs have revealed a mixture of compact and extended radio structures. Several of the latter are double-lobed/edge-brightened, consistent with the idea that such structures could be connected with the radiatively-efficient AGN accretion mode.