Galaxies at z ∼ 6:: The UV luminosity function and luminosity density from 506 HUDF, HUDF parallel ACS field, and goods i-dropouts

We have detected 506 i-dropouts (z similar to 6 galaxies) in deep, wide-area HSTACS fields: HUDF, enhanced GOODS, and HUDF parallel ACS fields (HUDF-Ps). The contamination levels are less than or similar to 8% (i.e., greater than or similar to 92% are at z similar to 6). With these samples, we present the most comprehensive, quantitative analyses of z similar to 6 galaxies yet and provide optimal measures of the UV luminosity function (LF) and luminosity density at z similar to 6, and their evolution to z similar to 3. We redetermine the size and color evolution from z similar to 6 to z similar to 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 (similar to 1350 angstrom) LF at z similar to 6 that extends to M-1350,M-AB similar to -17.5 (0.04L(z=3)*). There is strong evidence for evolution of the LF between z similar to 6 and z similar to 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 similar to 6. Density evolution (phi*) is ruled out at > 99.99% confidence. Despite large changes in the LF, the luminosity density at z similar to 6 is similar to (0.82 +/- 0.21 times) that at z similar to 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 similar to 6 the star formation rate density is just similar to 30% of the z similar to 3 value. Our UV LF is consistent with z similar to 6 galaxies providing the necessary UV flux to reionize the universe.