Galaxies at z~6: The UV Luminosity Function and Luminosity Density from   506 UDF, UDF-Ps, and GOODS i-dropouts
  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. 
R.J. Bouwens (UCSC), G.D. Illingworth (UCSC), J.P. Blakeslee (WSU), M.   Franx (Leiden)