Nature of the Matter Between the Molecular Torus and Accretion Disk

Determine in what form matter exists between the molecular torus and the accretion disk of an active galactic nucleus, including whether neutral molecular gas forms a dust-free broad-line-region component inside the dust-sublimation radius.

Background

The paper identifies an unresolved physical transition between the molecular and dusty torus surrounding the supermassive black hole and the innermost accretion disk. Although ionized broad-line-region gas and water masers have been observed, the authors state that the phase and structure of the material inside the dust-sublimation radius are not established. One proposed possibility is a neutral, molecular, dust-free broad-line region (BLR⁰), potentially organized in a thin, rapidly rotating disk and containing substantially more mass than the conventional ionized broad-line region. Detecting vibrationally excited CO and other molecular or atomic tracers with ALMA2040 is proposed as a way to test this possibility.