2010ApJ...718..657B

The Megamaser Cosmology Project (MCP) aims to determine H₀ by measuring angular-diameter distances to galaxies in the Hubble flow using observations of water vapor megamasers in the circumnuclear accretion disks of active galaxies. The technique is based only on geometry and determines H₀in one step, independent of standard candles and the extragalactic distance ladder. In Paper I, we presented a very long baseline interferometry map of the maser emission from the Seyfert 2 galaxy UGC 3789. The map reveals an edge-on, sub-parsec disk in Keplerian rotation, analogous to the megamaser disk in NGC 4258. Here, we present 3.2 years of monthly Green Bank Telescope observations of the megamaser disk in UGC 3789. We use these observations to measure the centripetal accelerations of both the systemic and high-velocity maser components. The measured accelerations suggest that maser emission lines near the systemic velocity originate on the front side of the accretion disk, primarily from segments of two narrow rings. Adopting a two-ring model for the systemic features, we determine the angular-diameter distance to UGC 3789 to be 49.9±7.0 Mpc. This is the most accurate geometric distance to a galaxy in the Hubble flow yet obtained. Based on this distance, we determine H₀= 69±11 km/s/Mpc. We also measure the mass of the central black hole to be 1.09x10⁷ M_☉±14%. With additional observations, the uncertainty in the distance to this galaxy can be reduced to under 10%. Observations of megamaser disks in other galaxies will further reduce the uncertainty in H₀ as measured by the MCP.