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We utilize the newly-acquired, ultra-deep WFC3/IR observations over the HUDFto search for star-forming galaxies at z~8-8.5, only 600 million years fromrecombination, using a Y_{105}-dropout selection. The new 4.7 arcmin**2 WFC3/IRobservations reach to ~28.8 AB mag (5 sigma) in the Y_{105}J_{125}H_{160}bands. These remarkable data reach ~1.5 AB mag deeper than the previous dataover the HUDF, and now are an excellent match to the HUDF optical ACS data. Forour search criteria, we use a two-color Lyman-Break selection technique toidentify z~8-8.5 Y_{105}-dropouts. We find 5 likely z~8-8.5 candidates. Thesources have H_{160}-band magnitudes of ~28.3 AB mag and very blue UV-continuumslopes, with a median estimated beta of <~-2.5 (where f_{\lambda}\propto\lambda^{\beta}). This suggests that z~8 galaxies are not only essentially dustfree but also may have very young ages or low metallicities. The observednumber of Y_{105}-dropout candidates is smaller than the 20+/-6 sourcesexpected assuming no evolution from z~6, but is consistent with the 5 expectedextrapolating the Bouwens et al. 2008 LF results to z~8. These results provideevidence 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 HST tocross a threshold, revealing star-forming galaxies at z~8-9.

Discovery of z~8 Galaxies in the HUDF from ultra-deep WFC3/IR Observations