Ionized Bubbles Throughout the Epoch of Reionization
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neyer-mneyer-phd-physics-2026-thesis.pdf
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Author(s)
Neyer, Meredith
Advisor(s)
Vogelsberger, Mark
Date Issued
May 2026
Publisher
Massachusetts Institute of Technology
Abstract
The Epoch of Reionization (EoR) marks the period roughly one billion years after the Big Bang during which the first galaxies ionized the neutral hydrogen of the intergalactic medium (IGM). While facilities such as the James Webb Space Telescope (JWST) and the Hydrogen Epoch of Reionization Array (HERA) are now delivering rich new observations of high-redshift galaxies and the redshifted 21 cm signal, the structure of the underlying neutral IGM remains exceptionally difficult to observe directly. The central aim of this thesis is to strengthen the quantitative connection between observable signatures and the three-dimensional structure of ionized regions, using the THESAN and LUMINA radiation-hydrodynamics simulation suites as the theoretical bridge between simulations and data.
I first develop a mean-free path ray-tracing analysis of THESAN that maps how the effective sizes of ionized bubbles correlate with the surrounding matter overdensity and the properties of nearby galaxies. Building on these relations, I construct a calibrated catalog of Lyman-alpha emitting galaxies (LAEs) in THESAN and identify the Lyman-alpha equivalent width as the most informative galactic probe of host bubble size, particularly at z > 7. I then move from one-dimensional sightline statistics to full three-dimensional morphology by applying a watershed segmentation algorithm to the 500 cMpc LUMINA simulation, characterizing the sizes, shapes, and internal properties of individual ionized bubbles and finding that they are preferentially prolate through most of reionization. Finally, in two collaborative projects building on these methods, we extend the framework to bubble merger histories in THESAN and to a 21 cm-LAE cross-correlation forecast carried out with the HERA team, demonstrating that statistical constraints on the reionization history are within reach of upcoming surveys.
Together, these results contribute to a coherent, simulation-informed framework for interpreting current and forthcoming observations of the first billion years of our Universe.
MIT Department
Massachusetts Institute of Technology. Department of Physics
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