The radio AGN population dichotomy: Green valley Seyferts versus red
  sequence low-excitation AGN
  Radio outflows of active galactic nuclei (AGN) are invoked in cosmological
models as a key feedback mechanism in the latest phases of massive galaxy
formation. Recently it has been suggested that the two major radio AGN
populations -- the powerful high-excitation, and the weak low-excitation radio
AGN (HERAGN and LERAGN, resp.) -- represent two earlier and later stages of
massive galaxy build-up. To test this, here we make use of a local (0.04<z<0.1)
sample of ~500 radio AGN with available optical spectroscopy, drawn from the
FIRST, NVSS, SDSS, and 3CRR surveys. A clear dichotomy is found between the
properties of low-excitation (absorption line AGN, and LINERs) and
high-excitation (Seyferts) radio AGN. The hosts of the first have the highest
stellar masses, reddest optical colors, and highest mass black holes but
accrete inefficiently (at low rates). On the other hand, the high-excitation
radio AGN have lower stellar masses, bluer optical colors (consistent with the
`green valley'), and lower mass black holes that accrete efficiently (at high
rates). Such properties can be explained if these two radio AGN populations
represent different stages in the formation of massive galaxies, and thus are
also linked to different phases of the `AGN feedback'.
