z~8 galaxies from ultra-deep WFC3/IR Observations over the HUDF
Abstract
We utilize the newly-acquired, ultra-deep WFC3/IR observations over the HUDF to search for star-forming galaxies at z~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 ~29 AB mag (5 sigma) in the Y_{105}J_{125}H_{160} bands. These remarkable data reach ~0.5 AB mag deeper than ever before, 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~8-8.5 Y_{105}-dropouts. We find 5 probable z~8-8.5 candidates. The sources have H_{160}-band magnitudes of ~28.5 AB mag, apparent sizes of ~0.15'' (~0.7 h^{-1} kpc), and very blue UV-continuum slopes (i.e., beta <~-2.5), suggesting that z~8 galaxies are not only dust free but also perhaps have very young ages or low metallicities. The observed number of Y_{105}-dropout candidates is smaller than the 21 and 10 sources expected assuming no evolution from z~6 and z~7, respectively, but is consistent with the 7 expected extrapolating the Bouwens et al. 2008 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 HST to cross a threshhold, revealing star-forming galaxies at z~8-9, and even possibly z~10 galaxies. The existence of such galaxies implies that substantial star formation occurred at even higher redshifts z>10, deep into JWST territory.Keywords
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