Sub-parsec viscous timescales drive FRII HERG to LERG transitions in radio-loud AGN
A single-author theoretical study places the FRII HERG-LERG transition at sub-parsec radii where viscous timescales and jet-induced heating jointly regulate accretion. The ~4 Myr transition arises naturally without external gas removal. Evidence remains model-dependent; direct kinematic mapping of molecular inflows is required to falsify the scenario.
The paper models the FRII HERG to LERG switch within the Garofalo-Evans-Sambruna framework, arguing that multiphase gas at broad-line-region and inner-torus radii throttles the mass supply once jet feedback heats the ISM. When the normalized accretion rate drops below ~0.01, the inner disk flips to an ADAF, quenching the high-excitation line emission while preserving the large-scale radio morphology. Molecular-gas observations in radio galaxies are cited as consistent because cold reservoirs remain but inward fueling is suppressed rather than exhausted.
This internally driven picture contrasts with merger-triggered or external-stripping scenarios that require longer or more stochastic timescales. It links jet power, accretion mode, and cosmic downsizing by placing the regulatory valve inside the sphere of influence where viscous times naturally match observed transition durations. The model therefore predicts that FRII LERGs should retain molecular gas masses comparable to HERGs but exhibit lower central concentrations on sub-parsec scales.
Future ALMA and ngVLA mapping of molecular tori in complete FRII samples at z < 0.3 can test the prediction by measuring inflow rates and comparing them against the critical m_crit threshold. If sub-parsec suppression is confirmed, semi-analytic models of radio-galaxy evolution will need to incorporate this radius-dependent feedback loop rather than treating accretion as a simple function of halo mass.
Sources (2)
- [1]Primary Source(https://arxiv.org/abs/2611.08497)
- [2]Supporting Source(https://arxiv.org/abs/1004.4891)