Modelling Molecular Gas and Its Tracers Across Cosmic Time

Abstract

Our understanding of the distant Universe and the processes governing galaxy formation and evolution largely stems from observing the light from stars and its interaction with the material surrounding them. However, an essential piece of this picture lies in the role of the interstellar medium (ISM) in shaping star formation within galaxies. Unveiling this aspect requires tracing the fuel for star formation – molecular gas. This thesis explores molecular gas in galaxies across cosmic time using cosmological simulations of galaxy formation. <br /> Simulating the molecular gas content of galaxies requires modelling various physical and chemical processes happening on a wide range of scales. On large scales, galaxy growth is affected by gas accretion from the cosmic web. On the other hand, molecular gas chemistry is regulated by conditions on small scales, which are beyond the resolving capabilities of large scale simulations needed to investigate the evolution of the cosmic molecular gas content. To tackle this multi-scale problem, we have developed a sub-grid model called HYACINTH – <em>HYdrogen And Carbon chemistry in the INTerstellar medium in Hydro simulations</em> – that can be embedded into large-scale cosmological simulations to track the abundances of molecular hydrogen (H₂), and its carbon-based observational proxies, namely, carbon monoxide (CO), atomic carbon (C), and singly-ionised carbon (C⁺), on the fly. <br /> We have implemented HYACINTH into the RAMSES code to perform the MARIGOLD simulations. Our simulated cosmic H2 density is in excellent agreement with current observational constraints. Additionally, we find that low-mass (M_H₂ < 10⁸ solar masses) galaxies contain nearly half of the cosmic H₂ in the early Universe. However, the sensitivity of current instruments renders these galaxies “invisible”, indicating a potential underestimation of the cosmic H₂ density in existing surveys. <br /> In recent years, the [CII] fine-structure line of C⁺ has emerged as a molecular gas tracer in the early Universe. As one of the brightest emission lines, it offers a unique window into the molecular ISM of distant galaxies, where conventional tracers like CO become observationally expensive. We tested the reliability of this line as a molecular gas tracer at different cosmic epochs. Our analysis reveals that the [CII]-molecular gas correlation is relatively weak in the first billion years of the Universe but grows in both strength and tightness over time. We further examined the time evolution of the [CII] luminosity function and the cosmic [CII] luminosity density, and found that faint (L[CII] < 10⁷ solar luminosities) galaxies contribute nearly half of the cosmic [CII] density in the early Universe. Overall, this thesis highlights the pivotal role of cosmological simulations in interpreting observations and providing crucial insights into the molecular gas reservoir of galaxies, that serves as the fuel for star formation across cosmic time.