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

TL;DR

This paper introduces a model of magnetically supported accretion disks in active galactic nuclei, explaining the formation of broad emission line regions and the suppression of star formation, thus addressing key issues in AGN fueling.

Contribution

It proposes a new class of magnetically elevated accretion disk models that naturally produce broad emission line regions and inhibit star formation, resolving longstanding puzzles in AGN theory.

Findings

Magnetic pressure elevates gas to large heights, forming broad emission line regions.

Local gravitational instability and star formation are suppressed in the model.

Star formation may occur in the equatorial zone at high mass supply rates.

Abstract

We propose that the accretion disks fueling active galactic nuclei are supported vertically against gravity by a strong toroidal ($\phi-$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 $z > 0.1 R$ 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.