Revisiting the Lyman Continuum Escape Crisis: Predictions for z > 6 from Local Galaxies

Abstract

The intrinsic escape fraction of ionizing Lyman continuum photons (f_(esc)) is crucial to understanding whether galaxies are capable of reionizing the neutral hydrogen in the early universe at z > 6. Unfortunately, it is not possible to access f_(esc) at z > 4 with direct observations, and the handful of measurements from low-redshift galaxies consistently find f_(esc) < 10%, while at least f_(esc) ∼ 10% is necessary for galaxies to dominate reionization. Here, we present the first empirical prediction of f_(esc) at z > 6 by combining the (sparsely populated) relation between [O III]/[O II] and f_(esc) with the redshift evolution of [O III]/[O II] as predicted from local high-z analogs selected by their Hα equivalent width. We find f_(esc) = 5.7(+8.3)(-3.3)% at z = 6 and f_(esc) = 10.4(+15.5)(-6.3)% at z = 9 for galaxies with log(M/M_⊙) ~ 9.0 (errors given as 1σ). However, there is a negative correlation with stellar mass and we find up to 50% larger f_(esc) per 0.5 dex decrease in stellar mass. The population-averaged escape fraction increases according to f_(esc) = f_(esc,0) ((1 + z) 3)^α, with f_(esc,0) = (2.3 ± 0.05)% and α = 1.17 ± 0.02 at z > 2 for log(M/M_⊙) ~ 9.0. With our empirical prediction of f_(esc) (thus fixing an important, previously unknown variable) and further reasonable assumptions on clumping factor and the production efficiency of Lyman continuum photons, we conclude that the average population of galaxies is just capable of reionizing the universe by z ∼ 6.

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