The population of radio AGN shows considerable diversity in both its AGN and its host galaxy properties. By investigating the causes of this diversity using a multi-wavelength approach, considerable recent progress has been made in understanding the underlying physical mechanisms.
Anisotropy and orientation. A broad range of observations now lend strong support to the unified schemes that explain the relationship between broad- and narrow-line radio AGN within the SLRG/FRII category in terms of anisotropy and orientation effects. The observations also demonstrate that two of the key indicators of the bolometric luminosities of the AGN — the [OIII] emission line luminosity (L[OIII]) and the 24µm continuum luminosity (L24) — suffer mild (factor ∼2 – 3) attenuation in NLRG, due to the extinction effects of the circum-nuclear dust.
Accretion rates. Differences in accretion rates onto the central supermassive black holes can further help to explain the diversity of the optical spectra and radio morphologies of the radio AGN population: the properties of SLRG/FRII sources are consistent with high Eddington accretion ratios, and those of WLRG/FRI sources with lower Eddington ratios, with the break between the two occurring at (Lbol + Qjet) / Ledd ∼ 10−2.
Variability. Long-term, high-amplitude variability of the AGN — either within a cycle, or in the switch-off phase at the end of a cycle — can help to explain the properties of the WLRG/FRII objects, whose relationship with the other classes of radio AGN is otherwise difficult to explain in terms of the effects of anisotropy/orientation and/or different accretion physics.
The triggering of SLRG. The detailed morphological, star formation, cool ISM and environmental properties of SLRG are consistent with them being triggered in galaxy mergers. However, the mergers are likely to be relatively minor in most cases: equivalent to the accretion of twice the gas mass of the LMC; only a minority of the hosts of radio AGN in the local universe (< 20%) have the high star formation rates and large cool ISM masses typical of major, gas-rich mergers that would lead to substantial growth of the supermassive black holes and stellar masses. Simulations of galaxy mergers indicate that timescale of this type of quasar-like radio AGN activity is likely to be relatively short (< 100 Myr: di Matteo, Springel & Hernquist, 2005).
The fuelling of WLRG. In contrast to the SLRG, the detailed properties of the WLRG are more consistent with their supermassive black holes being fuelled at low rates: by the accretion of the hot ISM in the host galaxies/clusters, the cool ISM from cooling flows, or cool ISM in the near-nuclear regions that has reached a relatively stable dynamical configuration. This type of radio AGN activity is likely to have a high duty cycle.