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 similar to 4 and similar to 5 and to extend our previous z similar to 6 sample. These samples contain 4671, 1416, and 627 B-, V-, and i-dropouts, respectively, and reach to extremely low luminosities [(0.1-0.4) L*(z=3) or M-UV similar to -16 to -17], allowing us to determine the rest-frame UV LF and faint-end slope alpha at z similar to 4-6 to high accuracy. We find faint-end slopes alpha = -1.73 +/- 0.05, -1.66 +/- 0.09, and -1.74 +/- 0.16 at z similar to 4, similar to 5, and similar to 6, respectively, suggesting that the faint-end slope is very steep and shows little evolution with cosmic time. We find that M*(UV) brightens considerably in the 0.7 Gyr from z similar to 6 to similar to 4 (by similar to 0.7 mag from M*(UV) = -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 phi* is not significant. The UV luminosity density at z similar to 6 is modestly lower than (0.45 +/- 0.09 times) that at z similar to 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 alpha found here suggest that lower luminosity galaxies play a significant role in reionizing the universe. Finally, recent search results for galaxies at z similar to 7-8 are used to extend our estimates of the evolution of M* from z similar to 7-8 to z similar to 4.