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Session 36 - Active Galactic Nuclei.
Display session, Tuesday, June 09
Atlas Ballroom,
Altogether, 20 active galaxies are known to harbor powerful water vapor masers towards their nuclei. The best studied and most fruitful of these is NGC 4258, in which the masers are found in a thin, rotating disk within about 0.25 pc from the central massive object. VLBI observations and single-dish monitoring of the maser combine to form a complete and elegant picture of the geometry and dynamics of molecular gas near the nucleus of this galaxy. Over the past four years we have periodically taken spectra of many of the other known masers with the intention of probing the gas dynamics and testing whether the time-dependent behavior of the masers in these galaxies resembles that in NGC 4258. Here we present the results of this monitoring program. In general, most masers do not appear to show velocity drifts with time as would be expected if they exist in an edge-on, rotating disk. The elegance of the maser in NGC 4258 is evidently rare among the known megamaser sources. We will discuss modifications to the thin disk model which might account for the monitoring data, and we will also discuss unusual changes in the maser spectra of individual sources.
