2025A&A...702A.156S

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.