2021MNRAS.502L..79I

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 x 10⁴ L_☉) 183 GHz H₂O 3_1,3-2_2,0 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 6_1,6-5_2,3 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.