A Scaling Relation Between Megamaser Disk Radius and Black Hole Mass in Active Galactic Nuclei

TL;DR

This paper finds an empirical linear relation between megamaser disk radius and black hole mass, explaining it through a model of molecular cloud capture and disk formation in active galactic nuclei.

Contribution

It introduces a model linking megamaser disk size to black hole mass via molecular cloud capture, explaining the observed linear correlation.

Findings

Empirical linear correlation between disk radius and black hole mass.

Disks form through partial capture of molecular clouds passing the black hole.

Disk properties depend on cloud column density, influencing stability and star formation.

Abstract

Several thin, Keplerian, sub-parsec megamaser disks have been discovered in the nuclei of active galaxies and used to precisely determine the mass of their host black holes. We show that there is an empirical linear correlation between the disk radius and the black hole mass. We demonstrate that such disks are naturally formed by the partial capture of molecular clouds passing through the galactic nucleus and temporarily engulfing the central supermassive black hole. Imperfect cancellation of the angular momenta of the cloud material colliding after passing on opposite sides of the hole leads to the formation of a compact disk. The radial extent of the disk is determined by the efficiency of this process and the Bondi-Hoyle capture radius of the black hole, and naturally produces the empirical linear correlation of the radial extent of the maser distribution with black hole mass. The disk has sufficient column density to allow X-ray irradiation from the central source to generate physical and chemical conditions conducive to the formation of 22 GHz H2O masers. For initial cloud column densities less than ~10^{23.5} cm^-2 the disk is non-self gravitating, consistent with the ordered kinematics of the edge-on megamaser disks; for higher cloud columns the disk would fragment and produce a compact stellar disk similar to that observed around Sgr A* at the galactic centre.