Delayed Photons from Binary Evolution Help Reionize the Universe
  High-resolution numerical simulations including feedback and aimed at
calculating the escape fraction (fesc) of hydrogen-ionizing photons often
assume stellar radiation based on single-stellar population synthesis models.
However, strong evidence suggests the binary fraction of massive stars is 70%.
Moreover, simulations so far yield values of fesc falling only on the lower end
of the roughly 10-20% range, the amount presumed necessary to reionize the
Universe. Analyzing a high-resolution (4 pc) cosmological radiation
hydrodynamic simulation we study how fesc changes when we include two different
products of binary stellar evolution - stars stripped of their hydrogen
envelopes and massive blue stragglers. Both produce significant amounts of
ionizing photons 10-200 Myr after each starburst. We find the relative
importance of these photons are amplified with respect to escaped ionizing
photons, because peaks in star formation rates (SFRs) and fesc are often out of
phase by this 10-200 Myr. Additionally, low mass, bursty galaxies emit Lyman
continuum radiation primarily from binary products when SFRs are low.
Observations of these galaxies by the James Webb Space Telescope could provide
crucial information on the evolution of binary stars as a function of redshift.
Overall, including stripped stars and massive blue stragglers increases our
photon-weighted mean escape fraction by around 13% and 10%, respectively,
resulting in a mean fesc of 17%. Our results emphasize that using updated
stellar population synthesis models with binary stellar evolution provides a
more sound physical basis for stellar reionization.
