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

Ma et al., available on arXiv

Abstract: 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 halos in M_vir~10^8–10^{12} M_sun (M_star~10^4–10^{10} M_sun) at z=5–12. We find the sample average increases with halo mass for M_vir~10^8–10^{9.5} M_sun, becomes nearly constant for M_vir~10^{9.5}–10^{11} M_sun, and decreases at M_vir>10^{11} M_sun. Equivalently, increases with stellar mass up to M_star~10^8 M_sun and decreases at higher masses. Even applying single-star stellar population synthesis models, we find a moderate ~0.2 for galaxies at M_star~10^8 M_sun. 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 by ~25–35% and the number of escaped photons by 60–80%. 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 at the high-mass end is due to dust attenuation, while at the low-mass end, decreases owing to inefficient star formation (and hence feedback). At fixed mass, tends to increase with redshift. Our simulations produce sufficient ionizing photons for cosmic reionization.