We propose a program to observe three water vapor lines at
high resolution in the five strongest known water vapor megamaser
galaxies: NGC1068, NGC3079, NGC4258, NGC4945 and Circinus.
The lines we have selected are: a bandhead line at 6.18 microns,
a major coolant line in shocked regions and a major maser pump
line in radiatively pumped maser models; the 7(25)-6(16) line at
29.837 microns, which connects to the upper level of the
powerful 1.3 cm maser transition; and the 6(33)-5(42) line at 194
microns, which is predicted to be a maser in theoretical models.
The emission at 6 microns in these galaxies is typically 1 - 3
Jy, of which half is from a stellar component and the rest from
non-stellar emission. We expect to detect the 6 micron line in
absorption against the compact non-thermal continuum emission. It
is possible, though unlikely, that we might detect the line in
emission from the maser region if it is radiatively pumped.
The 29 micron line links the upper level of the 1.3 cm maser line.
The strength of this line will allow, with reasonable modelling,
the population density of the maser levels to be determined.
Finally, a detection of maser emission at 194 microns would
significantly add to the understanding of the kinematics of the
subparsec regions in the nuclear disks of these galaxies.