Detection of maser emission at 183 and 380 GHz with ALMA in the gigamaser galaxy TXS 2226–184

Abstract

Context. The low-ionization nuclear emission-line region (LINER) galaxy TXS 2226−184 is known to host a very luminous 22 GHz water maser, called a gigamaser at the time of its discovery. To date, the nature of this maser is still being debated, in particular, whether it is associated with a nuclear accretion disk or with an ejection component, namely a jet or an outflow originating in the active galactic nucleus. Aims. We obtained multi-band (bands 5, 6, and 7) ALMA observations during Cycle 9, with the purpose of investigating the maser nature and the nuclear molecular material in the innermost region of the galaxy. Methods. While the full data sets are still under study, a preliminary data reduction and analysis of the band 5 and 7 spectral line cubes presented in this Letter already offer a significant outcome. Results. We observed bright, possibly maser emission from the water 183 GHz and 380 GHz transitions in TXS 2226−184. To the best of our knowledge, this represents the first unambiguous detection (S/N ≥ 100) of 380 GHz maser emission in a known 22-GHz maser galaxy, and the first case where all three transitions are present in the same object. Emission features at both frequencies show a two-peaked line profile resembling that of the 22 GHz maser features. The millimeter/submillimeter emission originates from a region coincident, within the errors, with that of the 22 GHz. Conclusions. The similarities in profile and position indicate that the emission at the three frequencies is likely produced by the same nuclear structure, although differences in line strengths and feature peak positions may hint at a slightly different physical conditions of the emitting gas. A comparison with the few megamaser sources studied at high enough detail and sharing similarities with the water lines in TXS 2226−184 favors a nature associated with the amplification of a bright nuclear continuum (from a jet or outflow) through dense and hot gas in front of the nucleus (e.g., a disk or torus); however, a more comprehensive analysis of the available data is necessary to better assess this scenario.