A Sub-Parsec Origin for Rapid Accretion-State Transitions in Radio-Loud AGN

Abstract

I investigate the physical origin and timescale of accretion-state transitions in radio-loud active galactic nuclei, focusing on the evolution from FRII high-excitation radio galaxies (HERGs) to FRII low-excitation radio galaxies (LERGs) within the framework of Garofalo, Evans & Sambruna (2010). I show that the required transition timescale of about 4 million yr arises naturally if the accretion rate is regulated at sub-parsec radii (about 0.1-0.3 pc), corresponding to viscous evolution at about one thousand to ten thousand R_g for black hole masses 100 million to 1 billion solar masses. These radii coincide with the broad-line region and inner dusty torus, where multiphase gas dynamics regulate the effective inward mass flux on Myr timescales. I argue that powerful FRII jet feedback heats and perturbs the surrounding medium, reducing the effective mass supply reaching the inner disk. Once the accretion rate falls below the critical threshold m_crit about 0.01, the inner flow rapidly transitions to an advection-dominated accretion flow (ADAF). Observations of molecular gas reservoirs in radio galaxies are compatible with this picture because the transition requires suppression of inward fueling rather than complete removal of cold gas. I conclude that rapid, internally driven evolution provides a physically consistent basis for the FRII HERG -> FRII LERG transition while linking radio morphology, accretion state, and cosmic evolution.