Abstract

We search for z similar to 10 galaxies over similar to 160 arcmin(2) 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 5 sigma depths ranging from 26.9 to 29.4 in H-160 AB mag. z greater than or similar to 9.5 galaxy candidates are identified via J(125) - H-160 > 1.2 colors and non-detections in any band blueward of J(125). Spitzer Infrared Array Camera (IRAC) photometry is key for separating the genuine high-z candidates from intermediate-redshift (z similar to 2-4) galaxies with evolved or heavily dust obscured stellar populations. After removing 16 sources of intermediate brightness (H-160 similar to 24-26 mag) with strong IRAC detections, we only find one plausible z similar to 10 galaxy candidate in the whole data set, previously reported in Bouwens et al.. The newer data cover a 3x larger area and provide much stronger constraints on the evolution of the UV luminosity function (LF). If the evolution of the z similar to 4-8 LFs is extrapolated to z similar to 10, six z similar to 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 similar to 8. The luminosity density is found to increase by more than an order of magnitude in only 170 Myr from z similar to 10 to z similar to 8. This increase is >= 4x larger 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 similar to 10.