Probing the Dawn of Galaxies at z ~ 9-12: New Constraints from HUDF12/XDF and CANDELS data

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.

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 ~ 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.

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