Galaxies in the Epoch of Reionization Are All Bark and No Bite -- Plenty of Ionizing Photons, Low Escape Fractions

Abstract: Early results from JWST suggest that epoch-of-reionization (EoR) galaxies produce copious ionizing photons, which, if they escape efficiently, could cause reionization to occur too early. We study this problem using JWST imaging and prism spectroscopy for 426 galaxies at 4.5 < z < 9.0. We fit these data simultaneously with stellar--population and nebular--emission models that include a parameter for the fraction of ionizing photons that escape the galaxy, f_esc. We find that the ionization production efficiency, $\xi_\mathrm{ion}$ = Q(H) / L(UV), increases with redshift and decreasing UV luminosity, but shows significant scatter, $\sigma( \log \xi_\mathrm{ion})$ ~= 0.3 dex. The inferred escape fractions averaged over the population are $\langle f_\mathrm{esc} \rangle$ = 3 +\pm 1% with no indication of evolution over 4.5 < z < 9.0. This implies that in our models nearly all of the ionizing photons need to be absorbed to account for the nebular emission. We compute the impact of our results on reionization, including the distributions for $\xi_\mathrm{ion}$ and f_esc, and the evolution and uncertainty of the UV luminosity function. Considering galaxies brighter than M(UV) < -16 mag, we would produce an IGM hydrogen-ionized fraction of x_e=0.5 at 5.5 < z < 7.1, possibly too late compared to constraints from from QSO sightlines. Including fainter galaxies, M(UV) < -14 mag, we obtain x_e = 0.5 at 6.7 < z < 9.6, fully consistent with QSO and CMB data. This implies that EoR galaxies produce plenty of ionizing photons, but these do not efficiently escape. This may be a result of high gas column densities combined with burstier star-formation histories, which limit the time massive stars are able to clear channels through the gas for ionizing photons to escape.