Oesch, P. A., Bouwens, R. J., Illingworth, G. D., Labbé, I., Franx, M., van Dokkum, P. G., Trenti, M., Stiavelli, M., Gonzalez, V., and Magee, D.
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 ~3 × and 9× larger than observed. The inferred star formation rate density (SFRD) in galaxies above 0.7 M &sun; yr-1 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 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 UV < -17.7 mag within only ~200 Myr from z ~ 10 to z ~ 8, in the heart of cosmic reionization.