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 <I>~</I> 10<FONT 
SIZE=-1><SUP>4</SUP></FONT> - 10<FONT SIZE=-1><SUP>7</SUP></FONT> 
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 (&gt; 10<FONT 
SIZE=-1><SUP>22</SUP></FONT> H atoms cm<FONT 
SIZE=-1><SUP>-2</SUP></FONT>) to the nucleus is required for 
detectable H<FONT SIZE=-1><SUB>2</SUB></FONT>O 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