The radio AGN population dichotomy: Green valley Seyferts versus red sequence low-excitation AGN

TL;DR

This study reveals a clear dichotomy between two types of radio AGN, with distinct host galaxy properties and accretion behaviors, supporting the idea they represent different evolutionary stages in galaxy formation.

Contribution

It provides observational evidence for the dichotomy between high- and low-excitation radio AGN and links these to different galaxy evolution phases.

Findings

Low-excitation AGN have high stellar masses and low accretion rates.

High-excitation AGN are found in bluer, less massive hosts with high accretion rates.

The two populations likely represent different stages of galaxy evolution.

Abstract

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'.