  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 modelled systematically
and quantitatively. After corrections, we derive a rest-frame continuum UV
(~1350 A) 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) though the
degree depends upon the faint-end slope. As expected from hierarchical models,
the most luminous galaxies are deficient at z~6. Density evolution (phi*) is
ruled out at >99.99% confidence. Despite large changes in the LF, the
luminosity density at z~6 is similar (0.82+/-0.21x) to 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 the true evolution is substantially larger: at z~6 the star
formation rate density is just ~30% of the z~3 value. Our UV luminosity
function is consistent with z~6 galaxies providing the necessary UV flux to
reionize the universe.
