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

TL;DR

This study reports the first detection of 183 GHz H2O maser emission in a distant galaxy using ALMA, revealing new insights into the molecular environment near an active supermassive black hole at z ~ 0.06.

Contribution

The paper presents the first observation of millimetre 183 GHz H2O maser emission in a galaxy at z ~ 0.06, demonstrating ALMA's capability to probe SMBH environments at large distances.

Findings

Detection of luminous 183 GHz H2O maser emission in the Superantennae galaxy.

H2O emission is spatially compact and likely originates from maser amplification near the AGN.

ALMA can detect millimetre H2O masers out to >270 Mpc, enabling studies of SMBH environments beyond the local universe.

Abstract

We present the results of 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, HCO+, and HNC J = 2-1), we detect a highly luminous (~6e4Lsun) 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.