Stellar population effects on the inferred photon density at reionization

The relationship between stellar populations and the ionizing flux with which they irradiate their surroundings has profound implications for the evolution of the intergalactic medium (IGM). We quantify the ionizing flux arising from synthetic stellar populations which incorporate the evolution of interacting binary stars. We determine that these show ionizing flux boosted by 60 per cent at 0.05 ≤ Z ≤ 0.3 Z⊙ and a more modest 10–20 per cent at near-solar metallicities relative to star-forming populations in which stars evolve in isolation. The relation of ionizing flux to observables such as 1500 Å continuum and ultraviolet spectral slope is sensitive to attributes of the stellar population including age, star formation history and initial mass function (IMF). For a galaxy forming 1 M⊙ yr−1, observed at >100 Myr after the onset of star formation, we predict a production rate of photons capable of ionizing hydrogen, Nion = 1.4 × 1053 s−1 at Z = Z⊙ and 3.5 × 1053 s−1 at 0.1 Z⊙, assuming a Salpeter-like IMF. We evaluate the impact of these issues on the ionization of the IGM, finding that the known galaxy populations can maintain the ionization state of the Universe back to z ∼ 9, assuming that their luminosity functions continue to MUV = −10, and that constraints on the IGM at z ∼ 2–5 can be satisfied with modest Lyman-continuum photon escape fractions of 4–24 per cent depending on assumed metallicity.