Escape fraction of ionizing photons during reionization: effects due to
  supernova feedback and runaway OB stars
  The fraction of hydrogen ionizing photons escaping from galaxies into the
intergalactic medium is a critical ingredient in the theory of reionization. We
use two zoomed-in, high-resolution (4 pc), cosmological radiation hydrodynamic
simulations with adaptive mesh refinement to investigate the impact of two
physical mechanisms (supernova feedback and runaway OB stars) on the escape
fraction (f_esc) at the epoch of reionization (z>7). We implement a new,
physically motivated supernova feedback model that can approximate the Sedov
solutions at all (from the free expansion to snowplow) stages. We find that
there is a significant time delay of about ten million years between the peak
of star formation and that of escape fraction, due to the time required for the
build-up and subsequent destruction of the star-forming cloud by supernova
feedback. Consequently, the photon number-weighted mean escape fraction for
dwarf galaxies in halos of mass 10^8-10^10.5 Msun is found to be <fesc>~11%,
although instantaneous values of f_esc>20% are common when star formation is
strongly modulated by the supernova explosions. We find that the inclusion of
runaway OB stars increases the mean escape fraction by 22% to <fesc>~14%. As
supernovae resulting from runaway OB stars tend to occur in less dense
environments, the feedback effect is enhanced and star formation is further
suppressed in halos with Mvir>10^9 Msun in the simulation with runaway OB stars
compared with the model without them. While both our models produce enough
ionizing photons to maintain a fully ionized universe at z>7 as observed, a
still higher amount of ionizing photons at z>9 appears necessary to accommodate
the high observed electron optical depth inferred from cosmic microwave
background observations.
