Searching Water Megamasers By Using Mid-infrared Spectroscopy (I):
  Possible Mid-infrared Indicators
  Water megamasers at 22 GHz with a gas disk configuration in galaxies provide
the most precise measurements of supermassive black hole masses, as well as
independent constraints on the Hubble constant in the nearby universe. The
existence of other maser types, such as jet or outflow masers, represents
another tracer for AGN science. However, the detection rate of water megamasers
in galaxies is extremely low. Over 40 years, only $\sim$ 160 galaxies are found
to harbour maser emission, and $\sim$ 30\% of them show features in their maser
emission that indicate a disk-like geometry. Therefore, increasing the
detection rate of masers is a crucial task to allow expanding on maser studies.
We present a comparison of mid-infrared spectroscopic data between a maser
galaxy sample and a Seyfert 2 control sample. We find that maser galaxies show
significant peculiarities in their mid-infrared spectra: (1) Maser galaxies
tend to present stronger silicate absorption at $\tau$ 9.7 $\mu$m than the
control sample, (2) PAH 11.3 $\mu$m emission in maser galaxies is much weaker
than in the control sample, (3) spectral indices at 20-30 $\mu$m are steeper in
maser galaxies than in the control sample and tend to be mid-infrared enhanced
population. We conclude that there may be good indicators in mid-infrared and
far-infrared which could differentiate maser and non-maser Seyfert 2 galaxies.
Upcoming infrared facilities, such as the James Webb Space Telescope, may be
able to exploit these and other useful criteria and tracers for water megamaser
observations.
