2009ApJ...690.1350O

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 ∼ 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 ∼ 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 ∼2-2.5 over the ∼ 170 Myr that separate z ∼ 6 and z ∼ 7, and a steady evolution for the LBG LF out to z ∼ 7, at the same rate that is observed throughout the z ∼ 3-6 period. This puts a strong constraint on the star-formation histories of z ∼ 6 galaxies, whose ensemble star-formation rate (SFR) density must be lower by a factor of 2 at ∼ 170 Myr before the epoch at which they are observed. In particular, a large fraction of stars in the z ∼ 6 LBG population must form at redshifts well above z ∼ 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 ∼ 6, provided that the faint-end slope of the z > 7 LF steepens to α ∼ -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 ∼2σ 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 ≳ 7, or the LF evolution must be compensated by a decrease in metallicity and a corresponding increase in ionization efficiency at these early epochs.