High-resolution ALMA observations of H₂S in LIRGs

Dense gas and shocks in outflows and circumnuclear disks

¹ Department of Space, Earth & Environment, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
² Department of Space, Earth and Environment, Onsala Space Observatory, Chalmers University of Technology, SE-439 92 Onsala, Sweden
³ Institute of Astronomy, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
⁴ Leiden Observatory, Leiden University, PO Box 9513 NL-2300 RA Leiden, The Netherlands
⁵ Transdisciplinary Research Area (TRA) ‘Matter’/Argelander-Institut für Astronomie, University of Bonn, Bonn, Germany
⁶ Physics and Astronomy, University College London, London, UK
⁷ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) Northwestern University, Evanston IL 60208, USA
⁸ Observatoire de Paris, LERMA, Collége de France, CNRS, PSL University, Sorbonne University, 75014 Paris, France
⁹ Observatorio Astronómico Nacional (OAN-IGN) – Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
¹⁰ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹¹ William H. Miller III Department of Physics and Astronomy, Johns Hopkins University, Baltimore MD 21218, USA
¹² Department of Astronomy, University of Wisconsin-Madison, 475 N Charter Street, Madison WI 53706, USA
¹³ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville VA 22903, USA
¹⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville VA 22903, USA
¹⁵ Section of Astrophysics, Astronomy, and Mechanics, Department of Physics, National and Kapodistrian University of Athens, Panepistimioupolis Zografou, 15784 Athens, Greece
¹⁶ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, FI-21500 Piikkiö, Finland

^⋆ Corresponding author: mamiko@chalmers.se

Received: 20 May 2025
Accepted: 14 August 2025

Abstract

Context. Molecular gas plays a critical role in regulating star formation and nuclear activity in galaxies. Sulphur-bearing molecules, such as H₂S, are sensitive to the physical and chemical environments in which they reside and are potential tracers of shocked, dense gas in galactic outflows and active galactic nuclei (AGNs).

Aims. We aim to investigate the origin of H₂S emission and its relation to dense gas and outflow activity in the central regions of nearby infrared-luminous galaxies.

Methods. We present Atacama Large Millimeter/submillimeter Array (ALMA) Band 5 observations of the ortho-H₂S 1_1, 0 − 1_0, 1 transition in three nearby galaxies: NGC 1377, NGC 4418, and NGC 1266. We performed radiative transfer modelling using RADEX to constrain the physical conditions of the H₂S-emitting gas and compare the results to ancillary CO and continuum data.

Results. We detect compact H₂S emission in all three galaxies, arising from regions smaller than ∼150 pc. The H₂S spectral profiles exhibit broad line wings, suggesting an association with outflowing or shocked gas. In NGC 4418, H₂S also appears to be tracing gas that is counter-rotating. A peculiar redshifted emission feature may be inflowing gas, or possibly a slanted outflow. RADEX modelling indicates that the H₂S-emitting gas has high densities (n_H2 ≳ 10⁷ cm⁻³) and moderately warm temperatures (40−200 K). The derived densities exceed those inferred from CO observations, implying that H₂S traces denser regions of the ISM.