  We present a series of high-resolution (20-2000 Msun, 0.1-4 pc) cosmological
zoom-in simulations at z~6 from the Feedback In Realistic Environment (FIRE)
project. These simulations cover halo masses 10^9-10^11 Msun and rest-frame
ultraviolet magnitude Muv = -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 (fesc) of
hydrogen ionizing photons. We find that the instantaneous fesc has large time
variability (0.01%-20%), while the time-averaged fesc over long time-scales
generally remains ~5%, considerably lower than the estimate in many
reionization models. We find no strong dependence of fesc 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
3-10 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 fesc 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 fesc 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
over-predict fesc.
