Abstract
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 velocities of 12 spectral features distributed throughout the 'low-velocity' complex, \V - V-o\ < 150 km s(-1), where the systemic velocity is V-o. Features in the high-velocity complexes (\V - V-o\ > 700 km s(-1)) drifted by less than similar to 1 km s(-1) yr(-1). Also, no perceptible velocity drift was observed in the megamaser spectra of NGC 3079 and NGC 1068. The NGC 4258 spectra reported here, in combination with VLBI data, 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 velocity 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) The high-velocity maser lines do not shaw significant acceleration and the emitting material must lie within 6 degrees 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 are that subtends 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. 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)/T-red similar to 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 (delta > -20 degrees) in far-infrared bright galaxies and active galaxies with compact nuclear radio structure, as well as M31 and M 101. In view of the apparent association of H2O megamaser emission with an ultra-compact nuclear structure in NGC 4258 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.