2013ApJ...773...75O

We present a comprehensive analysis of z > 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±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 ∼3xand 9x larger than observed. The inferred star formation rate density (SFRD) in galaxies above 0.7 M_☉/yr decreases by 0.6±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 10x 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_UV< -17.7 mag within only ∼200 Myr from z ∼ 10 to z ∼ 8, in the heart of cosmic reionization.