The difficulty of getting high escape fractions of ionizing photons from high-redshift galaxies: A view from the FIRE cosmological simulations

We present a series of high-resolution (20-2000M_⊙, 0.1-4 pc) cosmological zoom-in simulations at z ≳ 6 from the Feedback In Realistic Environment (FIRE) project. These simulations cover halo masses 10⁹-10¹¹M_⊙ and rest-frame ultraviolet magnitude M_UV = -9 to -19. These simulations include explicit models of the multi-phase ISM, star formation, and stellar feedback, which produce reasonable galaxy properties at z = 0-6. We post-process the snapshots with a radiative transfer code to evaluate the escape fraction (f_esc) of hydrogen ionizing photons. We find that the instantaneous f_esc has large time variability (0.01-20 per cent), while the time-averaged f_esc over long time-scales generally remains ≲5 per cent, considerably lower than the estimate in many reionization models. We find no strong dependence of f_esc on galaxy mass or redshift. In our simulations, the intrinsic ionizing photon budgets are dominated by stellar populations younger than 3 Myr, which tend to be buried in dense birth clouds. The escaping photons mostly come from populations between 3 and 10 Myr, whose birth clouds have been largely cleared by stellar feedback. However, these populations only contribute a small fraction of intrinsic ionizing photon budgets according to standard stellar population models. We show that f_esc can be boosted to high values, if stellar populations older than 3 Myr produce more ionizing photons than standard stellar population models (as motivated by, e.g. models including binaries). By contrast, runaway stars with velocities suggested by observations can enhance f_esc by only a small fraction. We show that 'sub-grid' star formation models, which do not explicitly resolve star formation in dense clouds with n ≫ 1 cm-3, will dramatically overpredict f_esc.