Discovery of z ∼ 8 galaxies in the hubble ultra deep field from ultra-deep WFC3/IR observations

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 ∼ 8-8.5, only 600 million years from recombination, using a Y 105- dropout selection. The new 4.7 arcmin2 WFC3/IR observations reach to 28.8 ABmag (5σ) in the Y 105 J 125 H 160 bands. 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 ∼ 8-8.5Y 105-dropouts. We find five likely z ∼ 8-8.5 candidates. The sources have H 160-band magnitudes of 28.3 ABmag and very blue UV-continuum slopes, with a median estimated β of ≲-2.5 (where f λ λβ). 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 Y 105-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 etal. 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. © 2010. The American Astronomical Society. All rights reserved.