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Ultra-deep WFC3/IR observations on the HUDF from the HUDF09 program revealedjust one plausible z~10 candidate UDFj-39546284. UDFj-39546284 had all theproperties expected of a galaxy at z~10 showing (1) no detection in the deepACS+WFC3 imaging data blueward of the F160W band, exhibiting (2) a bluespectral slope redward of the break, and showing (3) no prominent detection indeep IRAC observations. The new, similarly deep WFC3/IR HUDF12 F160Wobservations over the HUDF09/XDF allow us to further assess this candidate.These observations show that this candidate, previously only detected at ~5.9sigma in a single band, clearly corresponds to a real source. It is detected at~5.3 sigma in the new H-band data and at ~7.8 sigma in the full 85-orbit H-bandstack. Interestingly, the non-detection of the source (<1 sigma) in the newF140W observations suggests a higher redshift. Formally, the best-fit redshiftof the source utilizing all the WFC3+ACS (and IRAC+K-band) observations is11.8+/-0.3. However, we consider the z~12 interpretation somewhat unlikely,since the source would either need to be ~20x more luminous than expected orshow very high-EW Ly-alpha emission (which seems improbable given the extensiveneutral gas prevalent early in the reionization epoch). Lower-redshiftsolutions fail if only continuum models are allowed. Plausible lower-redshiftsolutions require that the H-band flux be dominated by line emission such asHalpha or [OIII] with extreme EWs. The tentative detection of line emission at1.6 microns in UDFj-39546284 in a companion paper suggests that such emissionmay have already been found.

Photometric Constraints on the Redshift of z~10 candidate UDFj-39546284 from deeper WFC3/IR+ACS+IRAC observations over the HUDF