2010ApJ...709L.133B

We utilize the newly acquired, ultra-deep WFC3/IR observations over the Hubble Ultra Deep Field (HUDF) to search for star-forming galaxies at z ∼ 8-8.5, only 600 million years from recombination, using a Y₁₀₅-dropout selection. The new 4.7 arcmin² WFC3/IR observations reach to ∼28.8 AB mag (5σ) in the Y₁₀₅J₁₂₅H₁₆₀bands. These remarkable data reach ∼1.5 AB mag deeper than the previous data over the HUDF, and now are an excellent match to the HUDF optical ACS data. For our search criteria, we use a two-color Lyman break selection technique to identify z ∼ 8-8.5Y₁₀₅-dropouts. We find five likely z ∼ 8-8.5 candidates. The sources have H₁₆₀-band magnitudes of ∼28.3 AB mag and very blue UV-continuum slopes, with a median estimated β of ≲-2.5 (where f_λ{vprop} λ^β). This suggests that z ∼ 8 galaxies are not only essentially dust free but also may have very young ages or low metallicities. The observed number of Y₁₀₅-dropout candidates is smaller than the 20±6 sources expected assuming no evolution from z ∼ 6, but is consistent with the five expected extrapolating the Bouwens et al. luminosity function (LF) results to z ∼ 8. These results provide evidence that the evolution in the LF seen from z ∼ 7 to z ∼ 3 continues to z ∼ 8. The remarkable improvement in the sensitivity of WFC3/IR has enabled Hubble Space Telescope to cross a threshold, revealing star-forming galaxies at z∼ 8-9.