Analysis of H2O megamaser disks and supermassive black hole masses

We present a systematic analysis of 23 H2O megamaser disks using an axisymmetric mass model that explicitly separates the black hole mass (MBH) from the disk mass (Mdisk). The derived MBH values are 4.9 × 10⁵\, M–6.4 × 10⁷\, M, and the disk masses Mdisk are 2.0 × 10⁴\, M–4.6 × 10⁷\, M, with Mdisk / MBH ~ 0.04–5, where the larger MBH, the smaller Mdisk/MBH, and one-third of the maser disks have a very small mass and exhibit Keplerian rotation. The maser disks probe sub-parsec scales (0.02–1 pc, except IC 1481). Their inner radii lie 6–11 times farther out than the dust sublimation radius and show weak dependences on hard X-ray luminosity, while additional factors such as gas density also influence disk structure. The disks have aspect ratios H/r 0.02–0.6. More than half of the disks show gravitational instability (Q < 1) within r 1 pc, particularly among massive disk systems. The Eddington luminosity ratios LX/LE~ 10⁻⁵–10⁻¹ are one to five orders of magnitude smaller than disk-based mass accretion rates, favoring radiatively inefficient accretion flows such as ADAFs for NGC 4258, but requiring more accurate observational and theoretical consideration for the other galaxies to determine accretion disk regimes. Massive disk systems (Mdisk/MBH > 0.2) lie below the canonical MBH–σ _* relation, but their large mass accretion rates can drive evolution on to the relation within ~ 10⁹(α /0.1)⁻¹ yr, where α is the viscosity parameter. Low-mass disk systems, by contrast, already lie on the relation and evolve slower due to weaker accretion. These results suggest that the MBH–σ _* relation holds for black holes whose growth is largely complete.