Megamaser Disks Reveal a Broad Distribution of Black Hole Mass in Spiral
  Galaxies
  We use new precision measurements of black hole masses from water megamaser
disks to investigate scaling relations between macroscopic galaxy properties
and supermassive black hole (BH) mass. The megamaser-derived BH masses span
10^6-10^8 M_sun, while all the galaxy properties that we examine (including
stellar mass, central mass density, central velocity dispersion) lie within a
narrow range. Thus, no galaxy property correlates tightly with M_BH in ~L*
spiral galaxies. Of them all, stellar velocity dispersion provides the tightest
relation, but at fixed sigma* the mean megamaser M_BH are offset by -0.6+/-0.1
dex relative to early-type galaxies. Spiral galaxies with non-maser dynamical
BH masses do not show this offset. At low mass, we do not yet know the full
distribution of BH mass at fixed galaxy property; the non-maser dynamical
measurements may miss the low-mass end of the BH distribution due to inability
to resolve the spheres of influence and/or megamasers may preferentially occur
in lower-mass BHs.
