2009ApJ...695..287R

The Megamaser Cosmology Project (MCP) seeks to measure the Hubble Constant (H₀) in order to improve the extragalactic distance scale and constrain the nature of dark energy. We are searching for sources of H₂ O maser emission from active galactic nuclei with sub-pc accretion disks, as in NGC 4258, and following up these discoveries with very long baseline interferometric (VLBI) imaging and spectral monitoring. Here we present a VLBI map of the H₂ O masers toward UGC 3789, a galaxy well into the Hubble Flow. We have observed masers moving at rotational speeds up to 800 km/s at radii as small as 0.08 pc. Our map reveals masers in a nearly edge-on disk in Keplerian rotation about a 10⁷ M_☉ supermassive black hole. When combined with centripetal accelerations, obtained by observing spectral drifts of maser features (to be presented in Paper II), the UGC 3789 masers may provide an accurate determination of H₀, independent of luminosities and metallicity and extinction corrections.