Delayed Photons from Binary Evolution Help Reionize the Universe

TL;DR

This study shows that including binary stellar evolution products in simulations increases the estimated escape fraction of ionizing photons, suggesting a more accurate understanding of how the universe was reionized.

Contribution

It introduces the impact of binary stellar evolution products on escape fractions in cosmological simulations, improving the physical realism of reionization models.

Findings

Including binary products increases mean escape fraction by ~13-10%.

Binary evolution products produce significant ionizing photons 10-200 Myr after starbursts.

Results suggest binary evolution models are crucial for understanding cosmic reionization.

Abstract

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.