Understanding the Cosmological Evolution of Galaxies with Intensity Mapping

Citation

Sun, Guochao (2023) Understanding the Cosmological Evolution of Galaxies with Intensity Mapping. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/fzqw-2m61. https://resolver.caltech.edu/CaltechTHESIS:11052022-133519365

Abstract

The intensity mapping (IM) technique has been proven to be a powerful addition to the toolkit for understanding the cosmology and astrophysics behind cosmic structure formation. From the nearby universe to the epochs of cosmic dawn and reionization, by mapping the large-scale structure traced by a certain intensity field, IM provides an economical and a holistic view of the formation and evolution of galaxies in the cosmological text, in a way that is highly complementary to traditional methods based on individual galaxy detections. In this thesis, I present a number of theoretical perspectives on how the IM technique, especially line intensity mapping (LIM), can help us better understand the cosmological evolution of galaxies --- all the way to the intriguing era of first galaxy formation.

In the first part of this thesis, I use the Tomographic Ionized-carbon Mapping Experiment (TIME), a pathfinder for LIM observations of the 158-micron [CII] line emission from the epoch of reionization (EoR), as an example to demonstrate the aspects of high-redshift star-forming galaxies that can be practically studied with LIM. In Chapter 2, I elaborate the science cases of TIME for the investigations of the EoR using the redshifted [CII] line as a star formation rate tracer, and the cosmic molecular gas content near cosmic noon using CO rotational lines redshifted into the same observing bandpass. The results also guide the design of future phases of TIME. In Chapter 3, I introduce and demonstrate an effective masking strategy for the cleaning of line interlopers such as CO from the [CII] data TIME will measure. Using proxies of CO emitters built from stacking analysis of deep, near-infrared selected galaxies, it provides a practical solution to the notoriously challenging line confusion problem for LIM data analysis.

The second part of this thesis focuses on the concept of multi-tracer LIM, namely the synergies among LIM observations of multiple distinct tracers. Forward modeling and inference tools based on semi-analytic models and semi-numerical simulations are developed to explore and showcase the scientific potential of multi-tracer LIM. In Chapter 4, I describe a self-consistent, semi-analytic framework for modeling a variety of LIM signals from the multi-phase interstellar medium (ISM) of galaxies, and use it to illustrate the potential application of LIM to shed light on mean ISM properties of galaxies. In Chapters 5 and 6, I present a new semi-numerical simulation called LIMFAST that is developed for efficiently and self-consistently simulating a plethora of IM signals in the high-redshift universe. The LIMFAST code is particularly tailored for revealing the connection between the EoR and the first galaxy formation with multiple cosmological probes.

Finally, in the last part of thesis, I show two example case studies where the IM technique is applied to investigate the astrophysics of star formation in galaxies. In Chapter 7, I present an updated analysis of the contributions from star-forming galaxies at z≳5 to the observed cosmic near-infrared background. Imprints that reveal the formation histories of first stars, including the prospects for detecting them with the forthcoming space missions, are also studied. In Chapter 8, I describe a novel way to constrain the global star formation law of galaxies using LIM measurements of the baryonic acoustic oscillations.

As an emerging technique in observational cosmology, IM is no doubt still in its early days, promising exciting scientific returns while facing various practical challenges. Studies described in this thesis represent only a tiny fraction of the theoretical efforts from the community, but they pave the way for more follow-up investigations that will eventually turn IM into a truly rewarding endeavor.

Thesis Files