Abstract

The most pressing questions around Reionization -- e.g., whether its protagonists were a handful of bright galaxies (``the oligarchs") or numerous ultra-faint sources (``democratic reionization") -- hinge on the unknown ionizing photon escape fractions (LyC fesc) of star-forming galaxies. Efforts to understand the physics and statistics of LyC fesc have been stymied by our inability to construct pure samples of LyC leakers vs. non-leakers with generalizable selection functions. I will argue high-resolution Lyman-alpha (LyA) spectroscopy is the panacea to these issues. Using the X-SHOOTER LyA survey at z~2 (XLSz2) I will present clean stacks of leakers vs. non-leakers that show dramatic differences across ~1000-8000A -- these differences clarify how/why ionizing photons escape and identify the features that can be studied at z>6 to constrain fesc (e.g., MgII, O32, HeII). The leakers constitute half our survey, and have escape fractions ~50%. Building on these results, I will present a LyA-based framework for the ionizing emissivity from z~2-8 that both explains the puzzling flatness of the emissivity at z~2-6 as well as rapid reionization between z~6-8. I will end by previewing two Cycle 1 JWST programs I am leading -- these programs examine ionized bubbles towards the beginning and end of reionization, and will produce some of the deepest grism pointings in Cycle 1 to enable a variety of z~1-9 science.