On the fundamental dichotomy in the local radio-AGN population:
  accretion, evolution, and host galaxy properties
  A sample of 18286 radio-loud AGN is presented, constructed by combining the
SDSS DR7 with the NVSS and FIRST radio surveys. Using this sample, the
differences between `high-excitation' (or `quasar-mode'; HERG) and
`low-excitation' (`radio-mode'; LERG) radio galaxies are investigated. A
primary difference is the distinct nature of the Eddington-scaled accretion
rate onto their central black holes: HERGs typically have accretion rates
between 1 and 10% of Eddington, whereas LERGs predominatly accrete at a rate
below 1% Eddington. This is consistent with models where the population
dichotomy is caused by a switch between radiatively efficient and inefficient
accretion modes at low accretion rates. Local radio luminosity functions are
derived separately for the two populations, showing that although LERGs
dominate at low luminosity and HERGs above 1e26 W/Hz, examples of both classes
are found at all radio luminosities. Using the V/Vmax test it is shown that the
populations show differential cosmic evolution at fixed radio luminosity: HERGs
evolve strongly at all luminosities, while LERGs show weak or no evolution.
This suggests that the luminosity-dependent evolution of the radio luminosity
function is driven, at least in part, by the changing relative contributions of
these two populations with luminosity. The host galaxies of the sources are
also distinct: HERGs are typically of lower stellar mass, with lower black hole
masses, bluer colours and weaker 4000-Ang breaks indicating younger stellar
populations. These results offer strong support to the picture in which HERGs
are fuelled at high rates through radiative accretion disks by cold gas,
perhaps from mergers and interactions, while LERGs are fuelled via radiatively
inefficient flows at low accretion rates, often by gas associated with the hot
X-ray haloes of their host galaxy/cluster, as part of a radio-AGN feedback loop
(abridged).
