THE UDF05 FOLLOW-UP OF THE HUBBLE ULTRA DEEP FIELD. II. CONSTRAINTS ON REIONIZATION FROM Z-DROPOUT GALAXIES

We detect three (plus one less certain) z850-dropout sources in two separate fields (Hubble Ultra Deep Field and NICP34) of our UDF05 Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer images. These z similar to 7 Lyman-break Galaxy (LBG) candidates allow us to constrain the Luminosity Function (LF) of the star-forming galaxy population at those epochs. By assuming a change in only M(*) and adopting a linear evolution in redshift, anchored to the measured values at z similar to 6, the best-fit evolution coefficient is found to be 0.43 +/- 0.19 mag per unit redshift (0.36 +/- 0.18, if including all four candidates), which provides a value of M*(z = 7.2) = -19.7 +/- 0.3. This implies a drop in the luminosity density in LBGs by a factor of similar to 2 - 2.5 over the similar to 170 Myr that separate z similar to 6 and z similar to 7, and a steady evolution for the LBG LF out to z similar to 7, at the same rate that is observed throughout the z similar to 3 - 6 period. This puts a strong constraint on the star-formation histories of z similar to 6 galaxies, whose ensemble star-formation rate (SFR) density must be lower by a factor of 2 at similar to 170 Myr before the epoch at which they are observed. In particular, a large fraction of stars in the z similar to 6 LBG population must form at redshifts well above z similar to 7. The rate of ionizing photons produced by the LBG population consistently decreases with the decrease in the cosmic SFR density. Extrapolating this steady evolution of the LF out to higher redshifts, we estimate that galaxies would be able to reionize the universe by z similar to 6, provided that the faint-end slope of the z > 7 LF steepens to alpha similar to -1.9 and that faint galaxies with luminosities below the current detection limits contribute a substantial fraction of the required ionizing photons. This scenario, however, gives an integrated optical depth to electron scattering that is similar to 2 sigma below the Wilkinson Microwave Anisotropy Probe-5 measurement. Therefore, altogether, our results indicate that, should galaxies be the primary contributors to reionization, either the currently detected evolution of the galaxy population slows down at z greater than or similar to 7, or the LF evolution must be compensated by a decrease in metallicity and a corresponding increase in ionization efficiency at these early epochs.