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BIB-VERSION:: AST-PP-v1.0
ID:: epreps.stsci//prep1197
ENTRY:: March 9, 1998
TITLE:: Water Vapor Megamasers in Active Galactic Nuclei
SUBTITLE::
AUTHOR:: Wilson, Andrew S.
AFFIL:: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA and: Astronomy Department, University of Maryland, College Park, MD 20742, USA
DATE:: December 1997
JOURNAL:: To appear in:
SUBMITTED::
ACCEPTED::
OTHER_ACCESS::
COPYRIGHT:: Copyright 1997 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
LANGUAGE:: English
ABSTRACT::

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 ~ 10 SIZE=-1>⁴ - 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 (> 10 SIZE=-1>²² H atoms cm SIZE=-1>^-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.

END:: epreps.stsci//prep1197