Escape fraction of the ionizing radiation from starburst galaxies at high redshifts

Recent data indicate that the cosmic ultraviolet emissivity decreased with decreasing redshift z near the end of reionization. Lacking evidence for very massive early stars, this could signal a decline with time in the mass-averaged escape fraction of ionizing radiation from galaxies < f(esc)> at z greater than or similar to 6. We calculate the evolution of ionization fronts in dark matter haloes which host gas in hydrostatic equilibrium at its cooling temperature floor (T approximate to 10(4) K for atomic hydrogen). We find a high escape fraction only for the lowest mass haloes [with M < 10(8.7) M-circle dot at (1 + z) = 10], provided their star formation efficiency f(star) greater than or similar to 10(-3). Since the low-mass galaxy population is depleted by radiative feedback, we find that indeed < f(esc)> decreases with time during reionization.