2012ApJ...745..110O

We search for z ∼ 10 galaxies over ∼160 arcmin² of Wide-Field Camera 3 (WFC3)/IR data in the Chandra Deep Field South, using the public HUDF09, Early Release Science, and CANDELS surveys, that reach to 5σ depths ranging from 26.9 to 29.4 in H₁₆₀ AB mag. z ≳ 9.5 galaxy candidates are identified via J₁₂₅- H₁₆₀> 1.2 colors and non-detections in any band blueward of J₁₂₅. Spitzer Infrared Array Camera (IRAC) photometry is key for separating the genuine high-z candidates from intermediate-redshift (z ∼ 2-4) galaxies with evolved or heavily dust obscured stellar populations. After removing 16 sources of intermediate brightness (H₁₆₀∼ 24-26 mag) with strong IRAC detections, we only find one plausible z ∼ 10 galaxy candidate in the whole data set, previously reported in Bouwens et al.. The newer data cover a 3xlarger area and provide much stronger constraints on the evolution of the UV luminosity function (LF). If the evolution of the z ∼ 4-8 LFs is extrapolated to z ∼ 10, six z ∼ 10 galaxies are expected in our data. The detection of only one source suggests that the UV LF evolves at an accelerated rate before z ∼ 8. The luminosity density is found to increase by more than an order of magnitude in only 170 Myr from z ∼ 10 to z ∼ 8. This increase is ≥ 4xlarger than expected from the lower redshift extrapolation of the UV LF. We are thus likely witnessing the first rapid buildup of galaxies in the heart of cosmic reionization. Future deep Hubble Space Telescope WFC3/IR data, reaching to well beyond 29 mag, can enable a more robust quantification of the accelerated evolution around z ∼ 10.