[Previous] | [Session 51] | [Next]
L. Greenhill (Harvard-Smithsonian CfA)
Water masers in active galactic nuclei (AGN) trace the structure and dynamics of accretion disks 0.1 to 1 pc from supermassive black holes. Because the masers can be readily studied with high velocity and angular resolutions (<1 km s-1 and <1 milliarcsecond), the orientation, shape, and rotation curves of the accretion disks can be resolved, in regions where direct imaging at optical and infrared wavelengths is often precluded. As a result, robust and precise estimates of black hole masses are possible. Spectroscopic and interferometric monitoring of the masers also permits the detection of centripetal acceleration and proper motion of disk material in rotation. Accelerations and proper motions can each be used to estimate distances via largely geometric arguments that do not depend on the sources of systemic uncertainty common to measurements tied to Standard Candles.
Today, the detections of masers in 23 AGN have been published, disks have been well resolved in three cases between 4 and 15 Mpc, and a distance (good to <7%) obtained for one case. Ultimately, the masers in another three of these 23 AGN, at distances of 40-70 Mpc, will probably be used to trace the underlying accretion disks with high precision. A new generation of surveys intended to discover more masers in AGN has begun recently, and these may be expected to double roughly the number of known sources.
[Previous] | [Session 51] | [Next]
Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.