Magnetically elevated accretion discs in active galactic nuclei: broad emission-line regions and associated star formation

Abstract

We propose that the accretion disks fueling active galactic nuclei are supported vertically against gravity by a strong toroidal (φ−direction) magnetic field that develops naturally as the result of an accretion disk dynamo. The magnetic pressure elevates most of the gas carrying the accretion flow at R to large heights z ∼ > 0.1R and low densities, while leaving a thin dense layer containing most of the mass -but contributing very little accretion -around the equator. We show that such a disk model leads naturally to the formation of a broad emission line region through thermal instability. Extrapolating to larger radii, we demonstrate that local gravitational instability and associated star formation are strongly suppressed compared to standard disk models for AGN, although star formation in the equatorial zone is predicted for sufficiently high mass supply rates. This new class of accretion disk models thus appears capable of resolving two longstanding puzzles in the theory of AGN fueling: the formation of broad emission line regions and the suppression of fragmentation thought to inhibit accretion at the required rates. We show that the disk of stars that formed in the Galactic Center a few million years ago could have resulted from an episode of magnetically elevated accretion at ∼ > 0.1 of the Eddington limit.

Puffy Accretion Disks: Sub-Eddington, Optically Thick, and Stable

“…From the observational side, studies of AGNs have recognized the need for "elevated" disks, thicker than the standard thin disk solutions (Begelman & Silk 2017). Our models respond to this need, and correspond to the recent magnetized disk models of Mishra et al (2019).…”

Global Radiation Magnetohydrodynamic Simulations of sub-Eddington Accretion Disks around Supermassive Black Holes

“…Amplification of magnetic fields near the disk midplane is thought to be balanced by the escape of magnetic fields away from the midplane due to buoyancy. This magnetic elevated model is found to have a larger pressure scale height and does not subject to the thermal and viscous instabilities (Sadowski 2016), which have interesting implications for both X-ray binaries and AGNs (Begelman et al 2015;Begelman & Silk 2017;Dexter & Begelman 2019). However, assumptions in the magnetic elevated disk model have not been checked numerically.…”

Extreme AGN variability: evidence of magnetically elevated accretion?

“…Moreover, shearing-box simulations, which lack toroidal field-line curvature and should be more stable than global simulations with a strong toroidal field (Blaes & Balbus 1994), show no evidence of MRI suppression in the nonlinear state. It seems reasonable to guess that magnetically elevated discs can thicken to H/R 0.1 (Begelman & Silk 2017). In the following we will adopt the result of equation 6, as well as constant H/R = 0.1 for comparison.…”