New Observational Insights into the Astrophysics of Extremely Metal-Poor O Stars

Low-metallicity massive stars shape the ISM and evolution of dwarf galaxies. Theory predicts that the astrophysics of these stars diverges significantly from their higher-metallicity counterparts, with important implications for stellar feedback and the drivers of cosmic reionization at high redshift. However, few observations of individual metal-poor O stars exist to validate widely adopted models of their winds, evolution, and ionizing fluxes. I will present Hubble Space Telescope far-ultraviolet spectra of three hot and massive O stars in galaxies spanning 3-14% solar metallicity. These data reveal that the two most metal-poor stars are both fast rotators driving weak stellar winds, suggesting high ionizing photon production. I will then show Keck/KCWI optical IFU spectroscopy of the HII region that is powered by the only O star in the extremely metal-poor galaxy Leo P. The observed nebular emission agrees remarkably well with the shape and normalization of the model ionizing spectrum fit to observations of the star, which had never before been tested at such low metallicity. I will conclude by describing a new HST program that will dramatically expand the dataset available to anchor the low-metallicity stellar models necessary to study stellar feedback processes and ionizing photon production in dwarf galaxies, both locally and in the early