On Estimating the Mass of Keplerian Accretion Disks in H2O Maser
  Galaxies
  H2O maser disks with Keplerian rotation in active galactic nuclei offer a
clean way to determine accurate black hole mass and the Hubble constant. An
important assumption made in using a Keplerian H2O maser disk for measuring the
black hole mass and the Hubble constant is that the disk mass is negligible
compared to the black hole mass. To test this assumption, a simple and useful
model can be found in Hure et al. (2011). In this work, the authors apply a
linear disk model to a position-dynamical mass diagram and re-analyze
position-velocity data from H2O maser disks associated with active galactic
nuclei. They claim that a maser disk with nearly perfect Keplerian rotation
could have disk mass comparable to the black hole mass. This would imply that
ignoring the effects of disk self-gravity can lead to large systematic errors
in the measurement of black hole mass and the Hubble constant. We examine their
methods and find that their large estimated disk masses of Keplerian disks are
likely the result of their use of projected instead of 3-dimensional position
and velocity information. To place better constraints on the disk masses of
Keplerian maser systems, we incorporate disk self-gravity into a 3-dimensional
Bayesian modelling program for maser disks and also evaluate constraints based
on the physical conditions for disks which support water maser emission. We
find that there is little evidence that disk masses are dynamically important
at the ~<1% level compared to the black holes.
