The effects of binary stars on galaxies and metal-enriched gas during reionization

Binary stars are abundant in nearby galaxies, but are typically unaccounted for in simulations of the high redshift Universe. Stellar population synthesis models that include the effects of binary evolution result in greater relative abundances of ionizing photons that could significantly affect the ambient ionizing background during the epoch of hydrogen reionization, additionally leading to differences in galaxy gas content and star formation. We use hydrodynamic cosmological simulations including in situ multifrequency radiative transfer to evaluate the effects of a high binary fraction in reionization-era galaxies on traits of the early intergalactic medium and the abundance of H I and He II ionizing photons. We further extend this to analyze the traits of enriched gas. In comparing metrics generated using a fiducial simulation assuming single stars with one incorporating a high binary fraction, we find that binary stars cause H I reionization to complete earlier and at an accelerated pace, while also increasing the abundances of high-ionization metals (C IV and Si IV) in simulated absorption spectra while reducing the abundance of low-ionization states (O I, Si II, and C II). However, through increased photoheating of galactic and circumgalactic gas, they simultaneously reduce the rate of star formation in low-mass galaxies, slowing the ongoing process of enrichment and suppressing their own ionizing background. This potentially contributes to a slower He II reionization process at z ≥ 5, and further indicates that self-regulation of galaxies could be underestimated when neglecting binary stellar evolution.