The Molecular Gas Content of z<0.1 Radio Galaxies: Linking the AGN Accretion Mode to Host Galaxy Properties

TL;DR

This study investigates the molecular gas content in low- and high-excitation radio AGN, revealing significant differences that support their classification as distinct evolutionary stages of galaxy development.

Contribution

It provides the first comprehensive comparison of molecular gas masses and host galaxy properties between LERAGN and HERAGN at low redshift, linking AGN accretion modes to galaxy evolution.

Findings

HERAGN have ~7 times more molecular gas than LERAGN

HERAGN hosts are younger with lower stellar and black hole masses

HERAGN show higher black hole accretion efficiencies

Abstract

One of the main achievements in modern cosmology is the so-called `unified model', which successfully describes most classes of active galactic nuclei (AGN) within a single physical scheme. However, there is a particular class of radio-luminous AGN that presently cannot be explained within this framework -- the `low-excitation' radio AGN (LERAGN). Recently, a scenario has been put forward which predicts that LERAGN, and their regular `high-excitation' radio AGN (HERAGN) counterparts represent different (red sequence vs. green valley) phases of galaxy evolution. These different evolutionary states are also expected to be reflected in their host galaxy properties, in particular their cold gas content. To test this, here we present CO(1-0) observations toward a sample of 11 of these systems conducted with CARMA. Combining our observations with literature data, we derive molecular gas masses (or upper limits) for a complete, representative, sample of 21 z<0.1 radio AGN. Our results yield that HERAGN on average have a factor of ~7 higher gas masses than LERAGN. We also infer younger stellar ages, lower stellar, halo, and central supermassive black masses, as well as higher black hole accretion efficiencies in HERAGN relative to LERAGN. These findings support the idea that high- and low-excitation radio AGN form two physically distinct populations of galaxies that reflect different stages of massive galaxy build-up.