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Empirical constraints on reionization require galactic ionizing photon escapefractions fesc>20%, but recent high-resolution radiation-hydrodynamiccalculations have consistently found much lower values ~1-5%. While thesemodels include strong stellar feedback and additional processes such as runawaystars, they almost exclusively consider stellar evolution models based onsingle (isolated) stars, despite the fact that most massive stars are inbinaries. We re-visit these calculations, combining radiative transfer andhigh-resolution cosmological simulations from the Feedback in RealisticEnvironments (FIRE) project. For the first time, we use a stellar evolutionmodel that includes a physically and observationally motivated treatment ofbinaries (the BPASS model). Binary mass transfer and mergers enhance thepopulation of massive stars at late times (>3 Myr) after star formation, whichin turn strongly enhances the late-time ionizing photon production (especiallyat low metallicities). These photons are produced after feedback from massivestars has carved escape channels in the ISM, and so efficiently leak out ofgalaxies. As a result, the time-averaged "effective" escape fraction (ratio ofescaped ionizing photons to observed 1500 A photons) increases by factors 4-10,sufficient to explain reionization. While important uncertainties remain, weconclude that binary evolution may be critical for understanding the ionizationof the Universe.

Binary Stars Can Provide the "Missing Photons" Needed for Reionization