  It has been argued that low-luminosity dwarf galaxies are the dominant source
of ionizing radiation during cosmological reionization. The fraction of
ionizing radiation that escapes into the intergalactic medium from dwarf
galaxies with masses less than ~10^9.5 solar masses plays a critical role
during this epoch. Using an extensive suite of very high resolution (0.1 pc),
adaptive mesh refinement, radiation hydrodynamical simulations of idealized and
cosmological dwarf galaxies, we characterize the behavior of the escape
fraction in galaxies between 3 x 10^6 and 3 x 10^9 solar masses with different
spin parameters, amounts of turbulence, and baryon mass fractions. For a given
halo mass, escape fractions can vary up to a factor of two, depending on the
initial setup of the idealized halo. In a cosmological setting, we find that
the time-averaged photon escape fraction always exceeds 25% and reaches up to
80% in halos with masses above 10^8 solar masses with a top-heavy IMF. The
instantaneous escape fraction can vary up to an order of magnitude in a few
million years and tend to be positively correlated with star formation rate. We
find that the mean of the star formation efficiency times ionizing photon
escape fraction, averaged over all atomic cooling (T_vir > 8000 K) galaxies,
ranges from 0.02 for a normal IMF to 0.03 for a top-heavy IMF, whereas smaller,
molecular cooling galaxies in minihalos do not make a significant contribution
to reionizing the universe due to a much lower star formation efficiency. These
results provide the physical basis for cosmological reionization by stellar
sources, predominately atomic cooling dwarf galaxies.
