Abstract

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 similar to 8-8.5, only 600 million years from recombination, using a Y-105-dropout selection. The new 4.7 arcmin(2) WFC3/IR observations reach to similar to 28.8 AB mag (5 sigma) in the Y(105)J(125)H(160) bands. These remarkable data reach similar to 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 similar to 8-8.5Y(105)-dropouts. We find five likely z similar to 8-8.5 candidates. The sources have H-160-band magnitudes of similar to 28.3 AB mag and very blue UV-continuum slopes, with a median estimated beta of less than or similar to -2.5 (where f(lambda) proportional to lambda(beta)). This suggests that z similar to 8 galaxies are not only essentially dust free but also may have very young ages or low metallicities. The observed number of Y-105-dropout candidates is smaller than the 20 +/- 6 sources expected assuming no evolution from z similar to 6, but is consistent with the five expected extrapolating the Bouwens et al. luminosity function (LF) results to z similar to 8. These results provide evidence that the evolution in the LF seen from z similar to 7 to z similar to 3 continues to z similar to 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 similar to 8-9.