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 (HCN J  = 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 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. [ABSTRACT FROM AUTHOR]