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STScI Preprint #1197
Water vapor masers provide the only emission line from accretion disks in active galaxies that can be imaged directly. I review the properties of this emission, emphasising how it can be used to derive physical properties of the accretion disks on scales ~ 104 - 107 gravitational radii. These properties include the temperature, magnetic field, disk stability, mass accretion rate, processes involved in heating the disk, the degree of warping and the disk's orientation and spin direction. Surveys of active galaxies indicate that a high column density of gas (> 1022 H atoms cm-2) to the nucleus is required for detectable H2O maser emission. There is also a trend for detectable maser emission to be correlated with the mid-infrared flux of the galaxy. VLBI images of the masers are consistent with an edge-on rotating disk in 4 out of 5 galaxies mapped to date. However, of these 5 galaxies, only NGC 4258 shows satellite lines with a Keplerian rotation curve, definite centripetal acceleration of the systemic lines and a sub-pc scale radio continuum jet apparently originating from the dynamical center and directed perpendicular to the disk. The disks are often significantly warped.
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD 21218, USA
and: Astronomy Department, University of Maryland,
College Park, MD 20742, USA