The Abundance of Star-Forming Galaxies in the Redshift Range 8.5 to 12: New Results from the 2012 Hubble Ultra Deep Field Campaign

We present the results of the deepest search to date for star-forming galaxies beyond a redshift z~8.5 utilizing a new sequence of near-infrared Wide Field Camera 3 images of the Hubble Ultra Deep Field. This `UDF12' campaign completed in September 2012 doubles the earlier exposures with WFC3/IR in this field and quadruples the exposure in the key F105W filter used to locate such distant galaxies. Combined with additional imaging in the F140W filter, the fidelity of high redshift candidates is greatly improved. Using spectral energy distribution fitting techniques on objects selected from a deep multi-band near-infrared stack we find 7 promising z>8.5 candidates. As none of the previously claimed UDF candidates with 8.5<z<10 is confirmed by our deeper multi-band imaging, our campaign has transformed the measured abundance of galaxies in this redshift range. Although we recover the candidate UDFj-39546284 (previously proposed at z=10.3), it is undetected in the newly added F140W image, implying it lies at z=11.9 or is an intense emission line galaxy at z~2.4. Although no physically-plausible model can explain the required line intensity given the lack of Lyman alpha or broad-band UV signal, without an infrared spectrum we cannot rule out an exotic interloper. Regardless, our robust z ~ 8.5 - 10 sample demonstrates a luminosity density that continues the smooth decline observed over 6 < z < 8. Such continuity has important implications for models of cosmic reionization and future searches for z>10 galaxies with JWST.