ALMA detection of millimetre 183 GHz H2O maser emission in the Superantennae galaxy at z ∼ 0.06

We present the results of Atacama Large Millimeter/submillimeter Array (ALMA) band-5 (∼170 GHz) observations of the merging ultraluminous infrared galaxy, the ‘Superantennae’ (IRAS 19254−7245), at z = 0.0617, which has been diagnosed as containing a luminous obscured active galactic nucleus (AGN). In addition to dense molecular line emission (HCNJ = 2–1, HCO+J = 2–1, and HNC J = 2–1), we detect a highly luminous (∼6 × 10$^{4}\, \mathrm{L}_{\odot }$) 183 GHz H2O 31,3–22,0 emission line. We interpret the strong H2O emission as largely originating in maser amplification in AGN-illuminated dense and warm molecular gas, based on (1) the spatially compact (≲220 pc) nature of the H2O emission, unlike spatially resolved (≳500 pc) dense molecular emission, and (2) a strikingly different velocity profile from, and (3) significantly elevated flux ratio relative to, dense molecular emission lines. H2O maser emission, other than the widely studied 22 GHz 61,6–52,3 line, has been expected to provide important information on the physical properties of gas in the vicinity of a central mass-accreting supermassive black hole (SMBH), because of different excitation energy. We here demonstrate that with highly sensitive ALMA, millimetre 183 GHz H2O maser detection is feasible out to >270 Mpc, opening a new window to scrutinize molecular gas properties around a mass-accreting SMBH far beyond the immediately local Universe.