1995A&A...304...21G

From our monitoring of the H₂O megamaser spectrum of NGC4258 (M106) during 1984-1986 and 1993-1994, we measure a mean linear drift of 9.5±1.1km/s/yr, using an automated analysis, in the velocities of 12 spectral features distributed throughout the `low-velocity' complex, |V-V<sub>o</sub>|<150km/s, where the systemic velocity is V<sub>o</sub>. Features in the high-velocity complexes (|V-V<sub>o</sub>|>700km/s) drifted by less than ∼1km/s/yr. Also, no perceptible velocity drift was observed in the megamaser spectra of NGC3079 and NGC1068. The NGC4258 spectra reported here, in combination with VLBI data, give strong support to a model in which the H<sub>2</sub>O emission arises in an edge-on Keplerian disk of inner radius 0.13pc and rotation velocity ∼1100km/s that is bound by a mass of at least 10<sup>7</sup>M<sub>☉</sub>. We confirm three predictions of the Keplerian disk model. (1) The high-velocity maser lines do not show significant acceleration and the emitting material must lie within 6deg of the midline of the disk. (2) The low-velocity maser lines all exhibit very similar accelerations and the emitting material lies in a radially-thin arc that subtends ∼ 10deg, 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. The distribution of velocity drifts for the components of the `low-velocity' complex indicate that there are two groups of clumps, at slightly different galactocentric radii, which emit the observed maser radiation (r_blue/r_red∼1.1). Models that invoke elliptical orbits to explain the distribution of drift rates do not explain characteristics of the low- and high-velocity emission. The relative intensities of the low- and high-velocity maser components, and the absence of observed negative velocity drifts can be explained for unsaturated maser emission and maser amplification of the radio continuum emission associated with the central object whose gravity binds the disk. The positional 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 results from a search for new H₂O maser sources (δ>-20deg) in far-infrared bright galaxies and active galaxies with compact nuclear radio structure, as well as M31 and M101. In view of the apparent association of H₂O megamaser emission with an ultra-compact nuclear structure in NGC4258 and other sources, we speculate that the presence of H₂O megamasers is directly related to this ultra-compact structure, rather than to the galaxy's infrared luminosity. Thus H₂O (and not CO) might become an important tracer of the dense cool circumnuclear gas in distant radio loud active galaxies.