Theoretical investigations of multiphase gas in high-z galaxies and their comparison with observation

Casavecchia, Benedetta (2026): Theoretical investigations of multiphase gas in high-z galaxies and their comparison with observation. Dissertation, LMU München: Faculty of Physics

Abstract

Far-infrared [CII] 158 micron and [OIII] 88 micron transitions, together with rest-frame optical [OIII] lines, are among the brightest cooling lines detected by ALMA and JWST in galaxies within the first billion years of cosmic history. These emission lines provide key diagnostics of star formation activity, gas metallicity, ionisation state, morphology, and kinematics. Their interpretation, however, requires a robust theoretical framework capable of linking line emission to the underlying physical conditions of the gas. Cosmological hydrodynamical simulations play a central role in this effort, as they evolve representative volumes of the Universe while incorporating increasingly sophisticated models of gas physics, chemical enrichment, and stellar feedback. By modelling emission-line processes within these simulations and directly comparing them to observations, this work aims to constrain the physical mechanisms shaping the earliest galaxies. In the first part of this thesis I model [CII] emission within the COLDSim cosmological simulations, exploiting a fully coupled non-equilibrium chemical network to follow the evolution of primordial atomic and molecular species, including key metal elements such as carbon. The cosmic mass density evolution of neutral and singly ionised carbon is computed for the first time, providing a global view of metal enrichment at z > 6. At the galaxy scale, scaling relations linking [CII] luminosity to star formation rate and stellar mass are derived and calibrated across different numerical resolutions and simulation volumes. These relations exhibit a linear trend, with an amplitude that evolves with redshift. The predicted trends are compared with ALMA observations, and revised high-redshift calibrations are provided to support the interpretation of current and future datasets. Building upon the [CII] modelling framework, in the second part of the thesis I investigate which gas phase is traced by [CII] emission at the epoch of reionisation. I quantify the relative contributions of HII, HI, and H2 to the total [CII] luminosity within a cosmological context. I find that [CII] emission is predominantly associated with neutral atomic gas, while molecular gas contributes a smaller fraction of the total luminosity. I derive conversion factors to estimate atomic and molecular gas masses from [CII] observations and explore their dependence on galaxy properties such as metallicity, star formation rate, and stellar mass. Although [CII] primarily traces HI-dominated regions, its luminosity remains tightly correlated with the molecular gas reservoir, making it a robust indirect probe of H2 mass in high-redshift galaxies. In the final part of the thesis, I model [OIII] emission within the SPICE radiation-hydrodynamical simulations to investigate its dependence on stellar feedback during the Epoch of Reionization. By comparing three supernova feedback prescriptions, bursty-sn, smooth-sn, and hyper-sn, I explore how variations in feedback energetics affect metal enrichment, ionisation conditions, and the spatial distribution of ionised gas. I find that [OIII] luminosity is regulated by both the metal content and the fraction of gas at T > 10^4 K. While the bursty-sn model efficiently ionizes gas, it enriches galaxies less effectively by z = 5, reducing the abundance of bright [OIII]-emitting systems relative to smooth-sn and hyper-sn. Spatially resolved [OIII] emission further shows that smooth-sn generally produces more compact galaxies and slightly higher V/sigma values, although the differences remain within the scatter of the other two models. This work bridges cosmological simulations and emission-line observations, laying the groundwork for future studies that will couple detailed gas physics with the next generation of high-sensitivity and spatially resolved observations.

• Item Type: Theses (Dissertation, LMU Munich)
• Subjects: 500 Natural sciences and mathematics; 500 Natural sciences and mathematics > 530 Physics
• Faculties: Faculty of Physics
• Language: English
• Date of oral examination: 28. April 2026
• 1. Referee: Springel, Volker
• Signature of the printed copy: 0001/UMC 32044