The Megamaser Cosmology Project: I. VLBI observations of UGC 3789

The Megamaser Cosmology Project (MCP) seeks to measure the Hubble Constant (Ho) in order to improve the extragalactic distance scale and constrain the nature of dark energy. We are searching for sources of water maser emission from AGN 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 water 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 10^7 Msun 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 Ho, independent of luminosities and metallicity and extinction corrections.