The SPHINX Cosmological Simulations of the First Billion Years: the
  Impact of Binary Stars on Reionization
  We present the SPHINX suite of cosmological adaptive mesh refinement
simulations, the first radiation-hydrodynamical simulations to simultaneously
capture large-scale reionization and the escape of ionizing radiation from
thousands of resolved galaxies. Our $5$ and $10$ co-moving Mpc volumes resolve
haloes down to the atomic cooling limit and model the inter-stellar medium with
better than $\approx10$ pc resolution. The project has numerous goals in
improving our understanding of reionization and making predictions for future
observations. In this first paper we study how the inclusion of binary stars in
computing stellar luminosities impacts reionization, compared to a model that
includes only single stars. Owing to the suppression of galaxy growth via
strong feedback, our galaxies are in good agreement with observational
estimates of the galaxy luminosity function. We find that binaries have a
significant impact on the timing of reionization: with binaries, our boxes are
$99.9$ percent ionized by volume at $z\approx 7$, while without them our
volumes fail to reionize by $z=6$. These results are robust to changes in
volume size, resolution, and feedback efficiency. The escape of ionizing
radiation from individual galaxies varies strongly and frequently. On average,
binaries lead to escape fractions of $\approx 7-10$ percent, about $3.5$ times
higher than with single stars only. The higher escape fraction is a result of a
shallower decline in ionizing luminosity with age, and is the primary reason
for earlier reionization, although the higher integrated luminosity with
binaries also plays a sub-dominant role.
