PROBING THE DAWN OF GALAXIES ATz∼ 9-12: NEW CONSTRAINTS FROM HUDF12/XDF AND CANDELS DATA

present a comprehensive analysis of z gt 8 galaxies based on ultra-deep WFC3/IR data. We exploit all the WFC3/IR imaging over the Hubble Ultra-Deep Field from the HUDF09 and the new HUDF12 program, in addition to the HUDF09 parallel field data, as well as wider area imaging over GOODS-South. Galaxies are selected based on the Lyman break technique in three samples centered around z ~ 9, z ~ 10, and z ~ 11, with seven z ~ 9 galaxy candidates, and one each at z ~ 10 and z ~ 11. We confirm a new z ~ 10 candidate (with z = 9. 8 plusmn 0. 6) that was not convincingly identified in our first z ~ 10 sample. Using these candidates, we perform one of the first estimates of the z ~ 9 UV luminosity function (LF) and improve our previous constraints at z ~ 10. Extrapolating the lower redshift UV LF evolution should have revealed 17 z ~ 9 and 9 z ~ 10 sources, i. e. , a factor ~3 and 9 larger than observed. The inferred star formation rate density (SFRD) in galaxies above 0. 7 M ⊙ yr^-1 decreases by 0. 6 plusmn 0. 2 dex from z ~ 8 to z ~ 9, in excellent agreement with previous estimates. From a combination of all current measurements, we find a best estimate of a factor 10 decrease in the SFRD from z ~ 8 to z ~ 10, following (1 + z) ^-11. 4 ± 3. 1. Our measurements thus confirm our previous finding of an accelerated evolution beyond z ~ 8, and signify a very rapid build-up of galaxies with M ₔₕ lt -17. 7 mag within only ~200 Myr from z ~ 10 to z ~ 8, in the heart of cosmic reionization. Based on data obtained with the Hubble Space Telescope operated by AURA, Inc. , for NASA under contract NAS5-26555.