Constraints on the Ionizing Efficiency of the First Galaxies
  Observations of the Lyman-alpha forest and of high-redshift galaxies at
z~5-10 imply that there were just enough photons to maintain the universe in an
ionized state at z~5-6, indicating a "photon-starved" end to reionization. The
ionizing emissivity must have been larger at earlier times in order to yield
the extended reionization history implied by the electron scattering optical
depth constraint from WMAP. Here we address the possibility that a faint
population of galaxies with host halo masses of ~1e8-1e9 Msun dominated the
ionizing photon budget at redshifts of about z>9, due to their much higher
escape fractions. Such faint, early galaxies, would not have formed in ionized
regions due to suppression by heating from the UV background (UVB), and would
therefore not contribute to the ionizing background at z<6, after reionization
is complete. Our model matches: (1) the low escape fractions observed for
high-redshift galaxies, (2) the WMAP constraint of tau_es~0.09, (3) the low
values for the UVB at z<6, and (4) the observed star formation rate density
inferred from Lyman-break galaxies. A top heavy IMF from Pop III stars is not
required in this scenario. We compare our model to recent ones in the
literature that were forced to introduce an escape fraction that increases
strongly towards high redshift, and show that a similar evolution occurs
naturally if low mass galaxies possess high escape fractions.
