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The Megamaser Cosmology Project (MCP) seeks to measure the Hubble Constant(Ho) in order to improve the extragalactic distance scale and constrain thenature of dark energy. We are searching for sources of water maser emissionfrom AGN with sub-pc accretion disks, as in NGC 4258, and following up thesediscoveries with Very Long Baseline Interferometric (VLBI) imaging and spectralmonitoring. Here we present a VLBI map of the water masers toward UGC 3789, agalaxy well into the Hubble Flow. We have observed masers moving at rotationalspeeds up to 800 km/s at radii as small as 0.08 pc. Our map reveals masers in anearly edge-on disk in Keplerian rotation about a 10^7 Msun supermassive blackhole. When combined with centripetal accelerations, obtained by observingspectral drifts of maser features (to be presented in Paper II), the UGC 3789masers may provide an accurate determination of Ho, independent of luminositiesand metallicity and extinction corrections.

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