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

October 2016 • 2016ApJ...829...99F

Authors • Faisst, Andreas L.

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}-3.3+8.3 % at z = 6 and {f}{{esc}}={10.4}-6.3+15.5 % 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.


IPAC Authors


Andreas Faisst

Assistant Scientist