High-resolution ALMA observations of H2S in LIRGs: Dense gas and shocks in outflows and circumnuclear disks

Abstract

<p><b>Context</b><br></p><p><em></em>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).</p><p><em>Aims.</em> 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.</p><p><b>Methods</b><br></p><p><em></em>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.</p><p><b>Results</b><br></p><p><em></em>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 (<em>n</em>_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.</p>