Mamiko Sato,

Luminous infrared galaxies (LIRGs) are dusty galaxies undergoing a transient evolutionary phase marked by rapid growth through starbursts or active galactic nuclei (AGN). Radiative feedback from these processes is absorbed by dust and re-emitted in the infrared, while mechanical feedback drives outflows that expel gas and dust. Molecular outflows are particularly critical to study, as they directly impact the cold, dense gas reservoir responsible for star formation, sometimes accelerating it to velocities exceeding 1000~km~s$^{-1}$. However, fundamental questions about outflow formation and physical processes remain unresolved.

This thesis investigates hydrogen sulphide (H$_2$S) as a novel diagnostic for probing dense molecular gas conditions in dusty galaxies and their outflows, with a focus on outflow driving mechanisms. Sulphur-bearing molecules like H$_2$S are sensitive tracers of shocked, dense gas in AGN and outflows, offering unique insights into chemical and physical environments. Observations with the Atacama Pathfinder Experiment (APEX) and IRAM Northern Extended Millimetre Array (NOEMA) (Paper~I) reveal new H$_2$S detections in LIRGs. At the resolution of these data, H$_2$S abundance enhancements are not directly linked to outflows, but we identify a potential connection between dense gas reservoirs and feedback properties. The similar infrared--H$_2$S and infrared--H$_2$O correlations suggest shared emission origins, likely from warm gas in shocks or regions irradiated by star formation or AGN.

In Paper~II, we expand this analysis with ALMA Band~5 observations of the ortho-H$_2$S $1_{1,0}$--$1_{0,1}$ transition in NGC~1377, NGC~4418, and NGC~1266. Compact H$_2$S emission ($<$150~pc scales) is detected in all three galaxies, with broad line wings indicative of outflowing or shocked gas. NGC~4418 exhibits counterrotating H$_2$S kinematics and a peculiar redshifted feature, possibly tracing inflowing gas or a slanted outflow. Radiative transfer modelling (RADEX) constrains the H$_2$S-emitting gas to extreme densities ($n_{\mathrm{H}_2} \gtrsim 10^7$~cm$^{-3}$) and warm temperatures (40--200~K), surpassing densities inferred from CO. This confirms H$_2$S as a selective tracer of ultra-dense molecular gas, likely influenced by AGN or starburst-driven shocks.

In Paper~III, we focus on one galaxy NGC~4418, a dusty LIRG hosting a compact obscured nucleus (CON). To investigate further the possible counter-rotation, we analysed the ALMA data with higher spatial resolution (0".05). The possible interpretations of the observation results are discussed through kinematic modelling for H$_2$S emission line and comparison with other species.

In summary, this thesis demonstrates that H$_2$S is frequently enhanced in shock- and feedback-dominated regions associated with star formation or AGN activity within the heavily obscured environments of galaxies, highlighting a connection between nuclear dense gas reservoirs and molecular outflows in their central regions. It also presents evidence of complex nuclear gas dynamics, including inflow, outflow, and possible counter-rotation, in one of the studied galaxies. This work touches upon the possible origin of H$_2$S enhancement, the fate of the dense outflowing gas and the long-term implications for galaxy evolution, while the detail aspects will remain a matter of investigation in future studies. Altogether, these results provide new insights into the life cycle of gas in the inner regions of infrared-luminous galaxies and emphasise the importance of multi-species, high-resolution observations for investigating feedback and the chemical evolution of galaxies.