Submillimeter H$_2$O megamasers in NGC 4945 and the Circinus galaxy
  We present 321 GHz observations of five AGN from ALMA Cycle 0 archival data:
NGC 5793, NGC 1068, NGC 1386, NGC 4945, and the Circinus galaxy. Submillimeter
maser emission is detected for the first time towards NGC 4945, and we present
a new analysis of the submillimeter maser system in Circinus. None of the other
three galaxies show maser emission, though we have detected and imaged the
continuum from every galaxy. Both NGC 4945 and Circinus are known to host
strong ($\gtrsim 10$ Jy) 22 GHz megamaser emission, and VLBI observations have
shown that the masers reside in the innermost $\sim 1$ parsec of the galaxies.
The peak flux densities of the 321 GHz masers in both systems are substantially
weaker (by a factor of $\sim$100) than what is observed at 22 GHz, though the
corresponding isotropic luminosities are more closely matched (within a factor
of $\sim$10) between the two transitions. We compare the submillimeter spectra
presented here to the known 22 GHz spectra in both galaxies, and we argue that
while both transitions originate from the gaseous environment near the AGN, not
all sites are necessarily in common. In Circinus, the spectral structure of the
321 GHz masers indicates that they may trace the accretion disk at radii
interior to the 22 GHz masers. The continuum emission in NGC 4945 and NGC 5793
shows a spatial distribution indicative of an origin in the galactic disks
(likely thermal dust emission), while for the other three galaxies the emission
is centrally concentrated and likely originates from the nucleus.
