SIMBAD references

Aims. The aim of this work is to search Seyfert 2 galaxy NGC 4945, a well-known 22GHz water megamaser galaxy, for H₂O (mega)maser emission at 183GHz.
Methods. We used APEX SEPIA Band 5 (an ALMA Band 5 receiver on the APEX telescope) to perform the observations.
Results. We detected 183GHz H₂O maser emission towards NGC 4945 with a peak flux density of ∼3Jy near the galactic systemic velocity. The emission spans a velocity range of several hundred km/s. We estimate an isotropic luminosity of >1000L_☉, classifying the emission as a megamaser. A comparison of the 183GHz spectrum with that observed at 22GHz suggests that 183GHz emission also arises from the active galactic nucleus (AGN) central engine. If the 183GHz 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 (1.6x10⁵ Schwarzschild radii), i.e. closer than the water maser emission previously detected at 22GHz. This is only the second time 183GHz 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 183GHz H₂O 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 183GHz compared with that for the 22GHz megamaser. This indicates that millimetre/submillimetre H₂O masers can indeed be useful 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.