UCLA

The reionization of intergalactic hydrogen in the universe’s first billion years was likely driven by the first stars and galaxies, so its history encodes information about their prop- erties. But the timeline of reionization is not well-measured and it is still unclear whether galaxies alone can produce the required ionizing photons. In my thesis I have focused on two ways to use galaxies at our current observational frontiers to constrain reionization and high redshift galaxy evolution.

One tool is the UV luminosity function (LF), which traces the evolution of star-forming galaxies and their ionizing photons. Accurately measuring LFs and understanding their evolution are important for understanding the connections between galaxies and their dark matter halos. I developed a simple, but powerful, semi-analytic model for LF evolution assuming star formation is driven by dark matter accretion. This has proved remarkably consistent with observations, and implies the majority of star formation at high redshifts occurs in low mass galaxies. I also developed a technique to improve the accuracy of LF by accounting for gravitational lensing magnification bias.

Secondly, Lyman alpha (Lyα) emission from galaxies can probe the intergalactic medium (IGM) ionization state as Lyα photons are strongly attenuated by neutral hydrogen, but this requires disentangling physics on pc to Gpc scales. I developed a new forward-modeling Bayesian framework combining cosmological IGM simulations with interstellar medium models to infer the IGM neutral hydrogen fraction from observations of Lyα emission. My thesis presents new measurements of the neutral fraction at z ∼ 7 and z ∼ 8, which, along with other independent constraints, provide increasing evidence for the bulk of reionization occurring at z ∼ 6 − 8. This is consistent with reionization being driven by ultra-faint galaxies with a low average ionizing photon escape fraction. We also show that reionization impacts Lyα emission from different galaxy populations in different ways: UV bright galaxies living in overdense regions that reionize early have potentially visible Lyα even in a highly neutral IGM.

The upcoming James Webb Space Telescope was designed to observe galaxies at Cosmic Dawn. Throughout this thesis I have made predictions for what JWST may observe in the context of high redshift galaxy evolution.