2022A&A...660A..82S

Context. In order to understand the evolution and feedback of active galactic nuclei (AGN) and star formation, it is important to use molecular lines as probes of physical conditions and chemistry.
Aims. We use H₂S to investigate the impact of starburst and AGN activity on the chemistry of the molecular interstellar medium in luminous infrared galaxies. Specifically, our aim is to search for evidence of shock enhancement of H₂S related to galactic-scale mechanical feedback processes such as outflows.
Methods. Using the APEX single-dish telescope, we have observed the 1₁₀-1₀₁ transition of ortho-H₂S at 168 GHz towards the centres of 12 nearby luminous infrared galaxies. We have also observed the same line towards the ultra-luminous infrared galaxy Mrk 231 with the NOEMA interferometer.
Results. We detected H₂S towards NGC 253, NGC 1068, NGC 3256, NGC 4418, NGC 4826, NGC 4945, Circinus, M 83, and Mrk 231. Upper limits were obtained for NGC 1097, NGC 1377, and IC 860. We also detected line emission from HCN 2-1 in all galaxies in the APEX survey as well as HCO⁺, HNC, CH₃CN, CH₃OH, H₂CS, HOC⁺, and SO in several of the sample galaxies. Mrk 231 has a rich 2 mm molecular spectrum and, in addition to H₂S, we detect emission from HC₃N, CH₃OH, HC¹⁸O⁺, C₂S, and CH₃CCH. Four galaxies show elevated H₂S emission relative to HCN: Circinus, NGC 3256, NGC 4826, and NGC 4418. We suggest that the high line ratios are caused by elevated H₂S abundances in the dense gas. However, we do not find any clear connection between the H₂S/HCN line intensity ratio and the presence (or speed) of molecular outflows in the sample galaxies. Therefore, H₂S abundances do not seem to be globally affected by the large-scale outflows. In addition, the H₂S/HCN line ratio is not enhanced in the line wings compared to the line core in Mrk 231. This suggests that H₂S abundances do not increase in the dense gas in the outflow. However, we do find that the H₂S and HCN luminosities (L_H2_S and L_HCN) correlate well with the total molecular gas mass in the outflow, M_outflow(H₂), in contrast to L_CO and L_HCO+^_. We also find that the line luminosity of H₂S correlates with the total infrared luminosity in a similar way as that of H₂O.
Conclusions. We do not find any evidence of H₂S abundance enhancements in the dense gas due to galactic-scale outflows in our sample galaxies, nor in the high-resolution study of Mrk 231. We discuss possible mechanisms behind the suggested H₂S abundance enhancements in NGC 4418, Circinus, NGC 3256, and NGC 4826. These include radiative processes (for example X-rays or cosmic rays) or smaller-scale shocks. Further high-resolution and multi-transition studies are required to determine the cause behind the elevated H₂S emission in these galaxies. We suggest that L_H2_S serves as a tracer of the dense gas content, similar to L_HCN, and that the correlation between L_H2_S and M_outflow(H₂) implies a relation between the dense gas reservoir and the properties and evolution of the molecular feedback. This potential link requires further study since it holds important keys to our understanding of how the properties of molecular outflows relate to those of their host galaxies. Finally, the similar infrared-correlation coefficients between H₂S and H₂O may indicate that they originate in the same regions in the galaxy: warm gas in shocks or irradiated by star formation or an AGN.