2006ApJ...653...53B

We have detected 506 i-dropouts (z∼6 galaxies) in deep, wide-area HST ACS fields: HUDF, enhanced GOODS, and HUDF parallel ACS fields (HUDF-Ps). The contamination levels are ≲8% (i.e., ≳92% are at z∼6). With these samples, we present the most comprehensive, quantitative analyses of z∼6 galaxies yet and provide optimal measures of the UV luminosity function (LF) and luminosity density at z∼6, and their evolution to z∼3. We redetermine the size and color evolution from z∼6 to z∼3. Field-to-field variations (cosmic variance), completeness, flux, and contamination corrections are modeled systematically and quantitatively. After corrections, we derive a rest-frame continuum UV (∼1350 Å) LF at z∼6 that extends to M_1350,AB~-17.5 (0.04L^*_z=3). There is strong evidence for evolution of the LF between z∼6 and z∼3, most likely through a brightening (0.6±0.2 mag) of M^* (at 99.7% confidence), although the degree depends on the faint-end slope. As expected from hierarchical models, the most luminous galaxies are deficient at z∼6. Density evolution (φ*) is ruled out at >99.99% confidence. Despite large changes in the LF, the luminosity density at z∼6 is similar to (0.82±0.21 times) that at z∼3. Changes in the mean UV color of galaxies from z∼6 to z∼3 suggest an evolution in dust content, indicating that the true evolution is substantially larger: at z∼6 the star formation rate density is just ∼30% of the z∼3 value. Our UV LF is consistent with z∼6 galaxies providing the necessary UV flux to reionize the universe.