High resolution ALMA observations of H$_2$S in LIRGS (Dense gas and shocks in outflows and CNDs)

TL;DR

This study uses ALMA observations to analyze H2S emission in three nearby luminous galaxies, revealing dense, shocked gas regions associated with outflows, inflows, and complex kinematics in galaxy centers.

Contribution

It provides new high-resolution ALMA data on H2S in LIRGs, demonstrating H2S as a tracer of dense, shocked gas in galactic outflows and nuclear regions, with detailed physical conditions derived.

Findings

H2S emission is compact and associated with outflows and shocks.

H2S traces denser regions than CO, with densities >10^7 cm^-3.

Detection of complex kinematic features including counterrotation and possible inflows.

Abstract

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 (AGN). 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. We present 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 perform 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. We detect compact H2S emission in all three galaxies, arising from regions smaller than approximately 150 parsecs. 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 counterrotating. A peculiar red shifted emission feature may correspond to inflowing gas, or possibly a slanted outflow. RADEX modelling indicates that the H2S emitting gas has high densities (molecular hydrogen density greater than 10^7 cm^-3) and moderately warm temperatures (between 40 and 200 Kelvin). The derived densities exceed those inferred from CO observations, implying that H2S traces denser regions of the interstellar medium.