Binary Stars Can Provide the "Missing Photons" Needed for Reionization

TL;DR

Incorporating binary star evolution into models significantly increases the predicted escape fraction of ionizing photons from galaxies, potentially resolving the discrepancy between observed and simulated values necessary for cosmic reionization.

Contribution

This study introduces the first use of binary stellar evolution models in high-resolution cosmological simulations to assess their impact on ionizing photon escape fractions.

Findings

Binary evolution increases effective escape fractions by factors of 4-10.

Late-time massive stars in binaries produce more ionizing photons after feedback creates escape channels.

Binary models can explain the photon budget needed for reionization.

Abstract

Empirical constraints on reionization require galactic ionizing photon escape fractions fesc>20%, but recent high-resolution radiation-hydrodynamic calculations have consistently found much lower values ~1-5%. While these models include strong stellar feedback and additional processes such as runaway stars, they almost exclusively consider stellar evolution models based on single (isolated) stars, despite the fact that most massive stars are in binaries. We re-visit these calculations, combining radiative transfer and high-resolution cosmological simulations from the Feedback in Realistic Environments (FIRE) project. For the first time, we use a stellar evolution model that includes a physically and observationally motivated treatment of binaries (the BPASS model). Binary mass transfer and mergers enhance the population of massive stars at late times (>3 Myr) after star formation, which in turn strongly enhances the late-time ionizing photon production (especially at low metallicities). These photons are produced after feedback from massive stars has carved escape channels in the ISM, and so efficiently leak out of galaxies. As a result, the time-averaged "effective" escape fraction (ratio of escaped ionizing photons to observed 1500 A photons) increases by factors 4-10, sufficient to explain reionization. While important uncertainties remain, we conclude that binary evolution may be critical for understanding the ionization of the Universe.