| We report results of several large surveys for water maser emission among Active Galactic Nuclei with the 100-m Green Bank Telescope and the two NASA Deep Space Network 70-m antennas at Tidbinbilla, Australia and at Robledo, Spain. We detected 23 new sources, which resulted in a 60% increase in the number of then known nuclear water maser sources. Eight new detections show the characteristic spectral signature of emission from an edge-on accretion disk and therefore constitute good candidates for the determination of black hole mass and geometric distance. This increase in the number of known sources has enabled us to reconsider statistical properties of the resulting sample. For the 30 water maser sources with available hard X-ray data, we found a possible correlation between unabsorbed X-ray luminosity (2--10 keV) and total isotropic water maser luminosity of the form L2--10 ∝ L0.5+/-0.1H2O , consistent with the model proposed by Neufeld et al. (1994) in which X-ray irradiation of molecular accretion disk gas by the central engine excites the maser emission.; We mapped for the first time with Very Long Baseline Interferomatey (VLBI) the full extent of the pc-scale accretion disk in NGC 3079 as traced by water maser emission. Positions and line-of-sight velocities of maser emission are consistent with a nearly edge-on pc-scale disk and a central mass of ∼ 2 x 106 M⊙ enclosed within ∼ 0.4 pc. Based on the kinematics of the system, we propose that the disk is geometrically-thick, massive, subject to gravitational instabilities, and hence most likely clumpy and star-forming. The accretion disk in NGC 3079 is thus markedly different from the compact, thin, warped, differentially rotating disk in the archetypal maser galaxy NGC 4258. We also detect maser emission at high latitudes above the disk and suggest that it traces an inward extension of the kpc-scale bipolar wide-angle outflow previously observed along the galactic minor axis.; We also report the first VLBI map of the pc-scale accretion disk in NGC 3393. Water maser emission in this source appears to follow Keplerian rotation and traces a linear structure between disk radii of 0.36 and ∼ 1 pc. Assuming an edge-on disk and Keplerian rotation, the inferred central mass is (3.1+/-0.2) x 107 M⊙ enclosed within 0.36+/-0.02 pc, which corresponds to a mean mass density of ∼ 108.2 M⊙ pc-3. We also measured with the Green Bank Telescope centripetal acceleration within the disk, from which we infer the disk radius of 0.17+/-0.02 pc for the maser feature that is located along the line of sight to the dynamical center. This emission evidently occurs much closer to the center than the emission from the disk midline (0.17 vs. 0.36 pc), contrary to the situation in the two archetypal maser systems NGC 4258 and NGC 1068. |
