The Radio AGN Population Dichotomy: Green Valley Seyferts Versus Red Sequence Low-Excitation Active Galactic Nuclei

Radio outflows of active galactic nuclei (AGNs) 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 AGNs (HERAGN and LERAGN, respectively)—represent two earlier and later stages of massive galaxy buildup. To test this, here we make use of a local (0.04 < z < 0.1) sample of ~500 radio AGNs 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 AGNs. 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 AGNs 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.