Dense gas inflows and outflow-driven shocks in luminous infrared galaxies: Insights from H2S and other molecules
Doktorsavhandling, 2026
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$S enhancement, the fate of the dense outflowing gas and the long-term implications for galaxy evolution, while the detail aspects will remain a matter of investigation in future studies. Altogether, these results provide new insights into the life cycle of gas in the inner regions of infrared-luminous galaxies and emphasise the importance of multi-species, high-resolution observations for investigating feedback and the chemical evolution of galaxies.
This thesis investigates hydrogen sulphide (H$_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$S enhancement, the fate of the dense outflowing gas and the long-term implications for galaxy evolution, while the detail aspects will remain a matter of investigation in future studies. Altogether, these results provide new insights into the life cycle of gas in the inner regions of infrared-luminous galaxies and emphasise the importance of multi-species, high-resolution observations for investigating feedback and the chemical evolution of galaxies.
Författare
Mamiko Sato
Chalmers, Rymd-, geo- och miljövetenskap, Astronomi och plasmafysik
APEX and NOEMA observations of H<inf>2</inf>S in nearby luminous galaxies and the ULIRG Mrk 231: A possible relation between dense gas properties and molecular outflows
Astronomy and Astrophysics,;Vol. 660(2022)
Artikel i vetenskaplig tidskrift
High-resolution ALMA observations of H2S in LIRGs: Dense gas and shocks in outflows and circumnuclear disks
Astronomy and Astrophysics,;Vol. 702(2025)
Artikel i vetenskaplig tidskrift
Complex Nuclear Gas Flows in NGC 4418: H2S as a Tracer of Infall and Outflow - With Supporting Multi-Line Evidence from HNC, CH3OH, and H2O
Luminous infrared galaxies (LIRGs) are dusty galaxies undergoing a short but intense phase of evolution, during which rapid growth is powered by vigorous star formation and/or accretion onto a central black hole. Much of the energy released in their centres is absorbed by dust and re-emitted at infrared wavelengths, while powerful molecular outflows can expel gas from the nuclear regions. These outflows may regulate future star formation by removing or redistributing the dense gas that fuels it, but the physical origin, composition, and evolution of such feedback processes remain poorly understood.
This thesis explores hydrogen sulphide (H$_2$S) as a new tracer of dense, warm, and dynamically disturbed molecular gas in dusty galaxy nuclei. Using observations from the APEX single-dish telescope and high-resolution interferometric data from NOEMA and ALMA, H$_2$S emission is detected in several nearby LIRGs and a ULIRG. The results show that H$_2$S preferentially traces very dense gas, often associated with shocks, turbulence, and energetic feedback linked to star formation or active galactic nuclei. On galaxy-wide scales, H$_2$S luminosity correlates with the amount of molecular gas participating in outflows, suggesting a connection between dense gas reservoirs and large-scale feedback.
High-resolution ALMA observations show that, on smaller scales, the H$_2$S-emitting gas can probe complex nuclear dynamics revealing detailed fuelling and feedback processes. In the compact obscured nucleus of NGC~4418, the H$_2$S emission traces multiple kinematic components, including rotation, and signatures of inflow and outflow.
It is found that inflow velocities may reach as high as 100 kms$^{-1}$ only a few pc from the nucleus. Radiative-transfer modelling shows that the H$_2$S emission arises from very dense gas, exceeding densities inferred from CO, highlighting its value as a probe of the most deeply embedded regions of galaxy centres.
Together, these studies demonstrate that H$_2$S is a powerful diagnostic of dense gas affected by feedback in heavily obscured galaxy nuclei. By linking the physical conditions of nuclear dense gas to molecular outflows and accretion processes, this work provides new insights into how feedback shapes the evolution and future growth of infrared-luminous galaxies.
This thesis explores hydrogen sulphide (H$_2$S) as a new tracer of dense, warm, and dynamically disturbed molecular gas in dusty galaxy nuclei. Using observations from the APEX single-dish telescope and high-resolution interferometric data from NOEMA and ALMA, H$_2$S emission is detected in several nearby LIRGs and a ULIRG. The results show that H$_2$S preferentially traces very dense gas, often associated with shocks, turbulence, and energetic feedback linked to star formation or active galactic nuclei. On galaxy-wide scales, H$_2$S luminosity correlates with the amount of molecular gas participating in outflows, suggesting a connection between dense gas reservoirs and large-scale feedback.
High-resolution ALMA observations show that, on smaller scales, the H$_2$S-emitting gas can probe complex nuclear dynamics revealing detailed fuelling and feedback processes. In the compact obscured nucleus of NGC~4418, the H$_2$S emission traces multiple kinematic components, including rotation, and signatures of inflow and outflow.
It is found that inflow velocities may reach as high as 100 kms$^{-1}$ only a few pc from the nucleus. Radiative-transfer modelling shows that the H$_2$S emission arises from very dense gas, exceeding densities inferred from CO, highlighting its value as a probe of the most deeply embedded regions of galaxy centres.
Together, these studies demonstrate that H$_2$S is a powerful diagnostic of dense gas affected by feedback in heavily obscured galaxy nuclei. By linking the physical conditions of nuclear dense gas to molecular outflows and accretion processes, this work provides new insights into how feedback shapes the evolution and future growth of infrared-luminous galaxies.
Ämneskategorier (SSIF 2025)
Astronomi, astrofysik och kosmologi
DOI
10.63959/chalmers.dt/5761
ISBN
978-91-8103-304-5
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5761
Utgivare
Chalmers
Lecture Hall EC
Opponent: Professor, Cecilia Ceccarelli, Université Grenoble Alpes (UGA), Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), CNRS, Grenoble, France