Masers and Broad-Line Mapping Favor Magnetically-Dominated AGN Accretion Disks

TL;DR

This paper uses maser and broad-line region observations to show that supermassive black hole accretion disks are likely dominated by magnetic pressure, ruling out thermal and radiation pressure models at large radii.

Contribution

It provides new observational constraints that favor magnetically-dominated disk models over thermal or radiation pressure-supported ones.

Findings

Thermal and radiation pressure-dominated models predict unphysically large disk masses and temperatures.

Observations rule out classic $ m \alpha$-disk models supported by thermal pressure.

Magnetically-dominated models are consistent with all current observational constraints.

Abstract

We present a novel and powerful constraint on the physics of supermassive black hole (BH) accretion disks. We show that in the outer disk (radii $R \gtrsim 0.01\,$pc or $\gtrsim 1000\,R_{G}$), models supported by thermal or radiation pressure predict disk masses which are much larger than the BH mass and increase with radius - i.e. rapidly-rising, extremely non-Keplerian rotation curves. More generally, we show that any observational upper limit to the deviation from Keplerian potentials at these radii directly constrains the physical form of the pressure in disks. We then show that existing maser and broad line region (BLR) kinematic observations immediately rule out the classic thermal-pressure-dominated Shakura Sunyaev-like $\alpha$-disk model, and indeed rule out any thermal or radiation (or cosmic-ray) pressure-dominated disk, as the required temperatures and luminosities of the gas at large radii would exceed those observed by orders of magnitude. We show that models where the pressure comes entirely from turbulence (without thermal, radiation, or magnetic sources) could in principle be viable but would require turbulent Toomre $Q \gtrsim 100$, far larger than predicted by self gravitating/gravito-turbulent models. However, recently proposed models of magnetic pressure-dominated disks agree with all of the observational constraints. These magnetically-dominated models also appear to agree better with constraints on maser magnetic fields, compared to the other possibilities. Observations appear to strongly favor the hypothesis that the outer regions of BH accretion disks are in the 'hyper-magnetized' state.