No missing photons for reionization: moderate ionizing photon escape fractions from the FIRE-2 simulations

We present the escape fraction of hydrogen ionizing photons (f_esc) from a sample of 34 high-resolution cosmological zoom-in simulations of galaxies at z ≥ 5 in the Feedback in Realistic Environments project, post-processed with a Monte Carlo radiative transfer code for ionizing radiation. Our sample consists of 8500 haloes in M_vir ~ 10⁸-$10^{12}\, M_{\odot }$ (M_* ~ 10⁴-$10^{10}\, M_{\odot }$) at z = 5-12. We find the sample average <f_esc>increases with halo mass for M_vir ~ 10⁸-$10^{9.5}\, M_{\odot }$, becomes nearly constant for 10^9.5-$10^{11}\, M_{\odot }$, and decreases at ${\gtrsim}10^{11}\, M_{\odot }$. Equivalently, <f_esc> increases with stellar mass up to $M_{\ast }\sim 10^8\, M_{\odot }$ and decreases at higher masses. Even applying single-star stellar population synthesis models, we find a moderate <f_esc> ~ 0.2 for galaxies at $M_{\ast }\sim 10^8\, M_{\odot }$. Nearly half of the escaped ionizing photons come from stars 1-3 Myr old and the rest from stars 3-10 Myr old. Binaries only have a modest effect, boosting <f_esc> by ~25-35 per cent and the number of escaped photons by 60-80 per cent. Most leaked ionizing photons are from vigorously star-forming regions that usually contain a feedback-driven kpc-scale superbubble surrounded by a dense shell. The shell is forming stars while accelerated, so new stars formed earlier in the shell are already inside the shell. Young stars in the bubble and near the edge of the shell can fully ionize some low-column-density paths pre-cleared by feedback, allowing a large fraction of their ionizing photons to escape. The decrease of <f_esc> at the high-mass end is due to dust attenuation, while at the low-mass end, <f_esc> decreases owing to inefficient star formation and hence feedback. At fixed mass, <f_esc> tends to increase with redshift. Although the absolute <f_esc>does not fully converge with resolution in our simulations, the mass- and redshift-dependence of <f_esc> is likely robust. Our simulations produce sufficient ionizing photons for cosmic reionization.