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.
