UV Luminosity Functions at z~4, 5, and 6 from the Hubble Ultra Deep Field and Other Deep Hubble Space Telescope ACS Fields: Evolution and Star Formation History^*

We use the ACS BViz data from the HUDF and all other deep HST ACS fields (including the GOODS fields) to find large samples of star-forming galaxies at z ~ 4 and ~5 and to extend our previous z ~ 6 sample. These samples contain 4671, 1416, and 627 B-, V-, and i-dropouts, respectively, and reach to extremely low luminosities [(0.01-0.04)L or M_UV ~ -16 to -17], allowing us to determine the rest-frame UV LF and faint-end slope α at z ~ 4-6 to high accuracy. We find faint-end slopes α = -1.73 ± 0.05, -1.66 ± 0.09, and -1.74 ± 0.16 at z ~ 4, ~5, and ~6, respectively, suggesting that the faint-end slope is very steep and shows little evolution with cosmic time. We find that M brightens considerably in the 0.7 Gyr from z ~ 6 to ~4 (by ~0.7 mag from M = -20.24 ± 0.19 to -20.98 ± 0.10). The observed increase in the characteristic luminosity over this range is almost identical to that expected for the halo mass function, suggesting that the observed evolution is likely due to the hierarchical coalescence and merging of galaxies. The evolution in ϕ* is not significant. The UV luminosity density at z ~ 6 is modestly lower than (0.45 ± 0.09 times) that at z ~ 4 (integrated to -17.5 mag) although a larger change is seen in the dust-corrected SFR density. We thoroughly examine published LF results and assess the reasons for their wide dispersion. We argue that the results reported here are the most robust available. The extremely steep faint-end slopes α found here suggest that lower luminosity galaxies play a significant role in reionizing the universe. Finally, recent search results for galaxies at z ~ 7-8 are used to extend our estimates of the evolution of M* from z ~ 7-8 to z ~ 4.