H2S and dense gas in luminous infrared galaxies and their outflows

Licentiate thesis, 2022

Luminous infrared galaxies (LIRGs) are dusty galaxies that are going through an important and likely transient phase in their evolution. They experience rapid growth through bursts of star formation or an accreting supermassive black hole, an Active Galactic Nucleus (AGN). Radiative feedback from the AGN or starburst is absorbed by the dust and re-emitted in the infrared. Mechanical feedback in the form of outflows can effectively expel gas and dust from the central regions of the LIRGs. Thus, studying the physical processes and conditions of outflows is one of the key elements for a complete understanding of galaxy evolution. Especially molecular outflows are significant because cold and dense gas, that normally participates in star formation, can be highly affected by the presence of the central energy source. In some cases, the gas may even be accelerated to velocities over 1000 km s−1. Many fundamental questions about the formation or physical processes of these feedback mechanisms still remain to be answered. In this thesis, I put my attention to one molecule, hydrogen sulphide (H2S), as a new diagnostic tool to probe the physical conditions in the dense molecular gas in dusty galaxies and their outflows. One aim is to investigate if H2S can be used to study the outflow driving mechanisms. The observations with the Atacama Pathfinder Experiment (APEX) telescope and the IRAM Northern Extended Millimeter Array (NOEMA) are described in Paper I. In this paper, we present new detections of ground state H2S line emission in a sample of LIRGs, and compare intensities to other lines, such as HCN and HCO+ 2–1. At the observed resolution, we do not find any H2S abundance enhancements linked to the outflows. Instead, we find a possible relation between the dense gas reservoir and the properties and evolution of the molecular feedback. Another point we discuss is the similar infrared-correlation coefficient between H2S and H2O which may indicate a similar origin of their emission. For example from warm gas in shocks or in gas irradiated by star formation or an AGN. The next step is to look into the dense dusty galaxy nuclei with higher spatial resolution, and to proceed to radiative transfer modelling using multitransition lines of several molecular gas tracers, including H2S.