Understanding cosmic reionization requires the identification and characterization of early sources of hydrogenionizing photons. The 2012 Hubble Ultra Deep Field (UDF12) campaign has acquired the deepest infrared images with the Wide Field Camera 3 aboard Hubble Space Telescope and, for the first time, systematically explored the galaxy population deep into the era when cosmic microwave background (CMB) data indicate reionization was underway. The UDF12 campaign thus provides the best constraints to date on the abundance, luminosity distribution, and spectral properties of early star-forming galaxies. We synthesize the new UDF12 results with the most recent constraints from CMB observations to infer redshift-dependent ultraviolet (UV) luminosity densities, reionization histories, and electron scattering optical depth evolution consistent with the available data. Under reasonable assumptions about the escape fraction of hydrogen-ionizing photons and the intergalactic medium clumping factor, we find that to fully reionize the universe by redshift z ∼ 6 the population of star-forming galaxies at redshifts z ∼ 7-9 likely must extend in luminosity below the UDF12 limits to absolute UV magnitudes of M UV ∼ −13 or fainter. Moreover, low levels of star formation extending to redshifts z ∼ 15-25, as suggested by the normal UV colors of z 7-8 galaxies and the smooth decline in abundance with redshift observed by UDF12 to z 10, are additionally likely required to reproduce the optical depth to electron scattering inferred from CMB observations.

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 α 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.

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