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 $z\geq5$, and
further indicates that self-regulation of galaxies could be underestimated when
neglecting binary stellar evolution.
