Escape fraction of ionizing photons from high-redshift galaxies in
  cosmological SPH simulations
  Combing the three-dimensional radiative transfer (RT) calculation and
cosmological SPH simulations, we study the escape fraction of ionizing photons
(f_esc) of high-redshift galaxies at z=3-6. Our simulations cover the halo mass
range of M_h = 10^9 - 10^12 M_sun. We postprocess several hundred simulated
galaxies with the Authentic Radiative Transfer (ART) code to study the halo
mass dependence of f_esc. In this paper, we restrict ourselves to the transfer
of stellar radiation from local stellar population in each dark matter halo. We
find that the average f_esc steeply decreases as the halo mass increases, with
a large scatter for the lower mass haloes. The low mass haloes with M_h ~ 10^9
M_sun have large values of f_esc (with an average of ~ 0.4), whereas the
massive haloes with M_h ~ 10^11 M_sun show small values of f_esc (with an
average of ~ 0.07). This is because in our simulations, the massive haloes show
more clumpy structure in gas distribution, and star-forming regions are
embedded inside these clumps, making it more difficult for the ionizing photons
to escape. On the other hand, in low mass haloes, there are often conical
regions of highly ionized gas due to the shifted location of young star
clusters from the center of dark matter halo, which allows the ionizing photons
to escape more easily than in the high-mass haloes. By counting the number of
escaped ionizing photons, we show that the star-forming galaxies can ionize the
intergalactic medium at z=3-6. The main contributor to the ionizing photons is
the haloes with M_h < 10^10 M_sun owing to their high f_esc. The large
dispersion in f_esc suggests that there may be various sizes of H{\sc ii}
bubbles around the haloes even with the same mass in the early stages of
reionization. We also examine the effect of UV background radiation field on
f_esc using simple, four different treatment of UV background.
