The Colors of Galaxies at 4 < z < 8 and their Contribution to Reionization

We present recent results on the rest-frame ultraviolet (UV) colors of galaxies at high-redshift, and the contribution of these galaxies to the reionization of the intergalactic medium (IGM). Using a combination of deep and wide data from the CANDELS, HUDF09 and ERS programs, we find that galaxies at z = 7 appear to be dust free, and they become substantially dustier by z = 4. Faint galaxies at z = 7 appear very blue, but they are consistent with the colors of very blue local galaxies, thus there is no evidence for the presence of exotic stellar populations. We find that the observable galaxy population can sustain a fully ionized IGM at z = 6 if the escape fraction of ionizing photons (f(esc)) is 30%. If the luminosity function extends much fainter, then the required f(esc) is lowered to similar to 10%. Examining the constraint on the emission rate of ionizing photons from Ly alpha forest measurements, we find that if the luminosity function extends to M-UV = -13, f(esc) must be less than 13% at z = 6. This escape fraction can still sustain a reionized IGM at z = 6, and even at z = 7, but unless it rises substantially at z > 6, the IGM may be similar to 20 - 50% neutral by z = 8.