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

Recent data indicates that the cosmic UV 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 〈fesc 〉 at z> ∼ 6. We calculate the evolution of ionization fronts in dark matter halos which host gas in hydrostatic equilibrium at its cooling temperature floor (T ≈ 104 K for atomic hydrogen). We find a high escape fraction only for the lowest mass halos (with M < 108.7 M at (1 + z) = 10) provided their star formation efficiency f?> ∼ 10−3. Since the low-mass galaxy population is depleted by radiative feedback, we find that indeed 〈fesc 〉 decreases with time during reionization.