The innermost region of the water megamaser radio galaxy 3C403

The standard unified scheme of active galactic nuclei requires the presence of high column densities of gas and dust potentially obscuring the central engine. So far, few direct subarcsecond resolution studies of this material have been performed toward radio galaxies. The goal of this paper is to elucidate the nuclear environment of the prototypical X-shaped Fanaroff-Riley type II radio galaxy 3C403, the only powerful radio galaxy known to host a water megamaser. Very Large Array A-array and single-dish Green Bank and Effelsberg 1.3 cm measurements were performed to locate and monitor the water maser emission. Very Long Baseline Interferometry 6 cm continuum observations were taken to analyze the spatial structure of the nuclear environment at even smaller scales, while the CO J=1-0 and 2-1 transitions were observed with the IRAM 30-m telescope to search for thermal emission from a spatially extended, moderately dense gas component.[abridged]Comment: 11 pages, 12 figures, accepted by A&A. For a version with high quality figures, see http://erg.ca.astro.it/~atarchi/3C403

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.Comment: 18 pages, 3 figures, 4 table

The Discovery of Water Maser Emission from Eight Nearby Galaxies

Using the Green Bank Telescope, we conducted a ``snapshot'' survey for water maser emission toward the nuclei of 611 galaxies and detected eight new sources. The sample consisted of nearby (v < 5000 km/s) and luminous (M_B < -19.5) galaxies, some with known nuclear activity but most not previously known to host AGNs. Our detections include both megamasers associated with AGNs and relatively low luminosity masers probably associated with star formation. The detection in UGC 3789 is particularly intriguing because the spectrum shows both systemic and high-velocity lines indicative of emission from an AGN accretion disk seen edge-on. Based on six months of monitoring, we detected accelerations among the systemic features ranging from 2 to 8 km/s/yr, the larger values belonging to the most redshifted systemic components. High-velocity maser lines in UGC 3789 show no detectable drift over the same period. Although UGC 3789 was not known to be an AGN prior to this survey, the presence of a disk maser is strong evidence for nuclear activity, and an optical spectrum obtained later has confirmed it. With follow up observations, it may be possible to measure a geometric distance to UGC 3789.Comment: to appear in Astrophysical Journal, 1 May 200