CENTRIPETAL ACCELERATION WITHIN THE SUBPARSEC NUCLEAR MASER DISK OF NGC-4258

From our monitoring of the H2O megamaser spectrum of NGC 4258 (M 106) during 1984-1986 and 1993-1994, we measure a mean linear drift of 9.5 +/- 1.1 km s(-1) yr(-1), using an automated analysis, in the velocitie s of 12 spectral features distributed throughout the 'low-velocity' co mplex, \V - V-o\ < 150 km s(-1), where the systemic velocity is V-o. F eatures in the high-velocity complexes (\V - V-o\ > 700 km s(-1)) drif ted by less than similar to 1 km s(-1) yr(-1). Also, no perceptible ve locity drift was observed in the megamaser spectra of NGC 3079 and NGC 1068. The NGC 4258 spectra reported here, in combination with VLBI da ta, give strong support to a model in which the H2O emission arises in an edge-on Keplerian disk of inner radius 0.13 pc and rotation veloci ty similar to 1100 km s(-1) that is bound by a mass of at least 10(7) M(.). We confirm three predictions of the Keplerian disk model. (1) Th e high-velocity maser lines do not shaw significant acceleration and t he emitting material must lie within 6 degrees of the midline of the d isk. (2) The low-velocity maser lines all exhibit very similar acceler ations and the emitting material lies in a radially-thin are that subt ends similar to 10 degrees, along the near side inner edge of the disk . (3) In spite of the acceleration of low-velocity maser features, the overall velocity range of the complex remains stationary over time. T he distribution of velocity drifts for the components of the 'low-velo city' complex indicate that there are two groups of clumps, at slightl y different galactocentric radii, which emit the observed maser radiat ion (r(blue)/T-red similar to 1.1). Models that invoke elliptical orbi ts to explain the distribution of drift rates do not explain character istics of the low- and high-velocity emission. The relative intensitie s of the low- and high-velocity maser components, and the absence of o bserved negative velocity drifts can be explained for unsaturated mase r emission and maser amplification of the radio continuum emission ass ociated with the central object whose gravity binds the disk. The posi tional alignment of H2O and radio continuum emission, in NGC 4258 and other sources, is also suggestive of a link between line and continuum fluxes in nuclear masers (e.g., amplification) Also reported are resu lts from a search for new H2O maser sources (delta > -20 degrees) in f ar-infrared bright galaxies and active galaxies with compact nuclear r adio structure, as well as M31 and M 101. In view of the apparent asso ciation of H2O megamaser emission with an ultra-compact nuclear struct ure in NGC 4258 and other sources, we speculate that the presence of H 2O megamasers is directly related to this ultra-compact structure, rat her than to the galaxy's infrared luminosity. Thus H2O (and not CO) mi ght become an important tracer of the dense cool circumnuclear gas in distant radio loud active galaxies.