Abstract

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 similar to 9, z similar to 10, and z similar to 11, with seven z similar to 9 galaxy candidates, and one each at z similar to 10 and z similar to 11. We confirm a new z similar to 10 candidate (with z = 9.8 +/- 0.6) that was not convincingly identified in our first z similar to 10 sample. Using these candidates, we perform one of the first estimates of the z similar to 9 UV luminosity function (LF) and improve our previous constraints at z similar to 10. Extrapolating the lower redshift UV LF evolution should have revealed 17 z similar to 9 and 9 z similar to 10 sources, i.e., a factor similar to 3x and 9x larger than observed. The inferred star formation rate density (SFRD) in galaxies above 0.7M(circle dot) yr(-1) decreases by 0.6 +/- 0.2 dex from z similar to 8 to z similar to 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 similar to 8 to z similar to 10, following (1 + z)(-11.4 +/- 3.1). Our measurements thus confirm our previous finding of an accelerated evolution beyond z similar to 8, and signify a very rapid build-up of galaxies with M-UV -17.7 mag within only similar to 200 Myr from z similar to 10 to z similar to 8, in the heart of cosmic reionization.