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 ≳ 0.01 pc or ≳ 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 α -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 ≳ 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.
Hopkins et al. (Tue,) studied this question.