  Aim: The aim of this work is to search Seyfert 2 galaxy NGC 4945, a
well-known 22 GHz water megamaser galaxy, for water (mega)maser emission at 183
GHz. Method: We used APEX SEPIA Band 5 to perform the observations. Results: We
detected 183 GHz water maser emission towards NGC 4945 with a peak flux density
of ~3 Jy near the galactic systemic velocity. The emission spans a velocity
range of several hundred km/s. We estimate an isotropic luminosity of > 1000
Lsun, classifying the emission as a megamaser. A comparison of the 183 GHz
spectrum with that observed at 22 GHz suggests that 183 GHz emission also
arises from the active galactic nucleus (AGN) central engine. If the 183 GHz
emission originates from the circumnuclear disk, then we estimate that a
redshifted feature at 1084 km/s in the spectrum should arise from a distance of
0.022 pc from the supermassive black hole (1.6 x 10(5) Schwarzschild radii),
i.e. closer than the water maser emission previously detected at 22 GHz. This
is only the second time 183 GHz maser emission has been detected towards an AGN
central engine (the other galaxy being NGC 3079). It is also the strongest
extragalactic millimetre/submillimetre water maser detected to date.
Conclusions: Strong millimetre 183 GHz water maser emission has now been shown
to occur in an external galaxy. For NGC 4945, we believe that the maser
emission arises, or is dominated by, emission from the AGN central engine.
Emission at higher velocity, i.e. for a Keplerian disk closer to the black
hole, has been detected at 183 GHz compared with that for the 22 GHz megamaser.
This indicates that millimetre/submillimetre water masers can indeed be useful
probes for tracing out more of AGN central engine structures and dynamics than
previously probed. Future observations using ALMA Band 5 should unequivocally
determine the origin of the emission in this and other galaxies.
