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

Ultra-deep WFC3/IR observations on the HUDF from the HUDF09 program revealed just one plausible z 10 candidate, UDFj-39546284. UDFj-39546284 had all the properties expected of a galaxy at z 10 showing (1) no detection in the deep ACS+WFC3 imaging data blueward of the F160W band, exhibiting (2) a blue spectral slope redward of the break, and showing (3) no prominent detection in deep IRAC observations. The new, similarly deep WFC3/IR HUDF12 F160W observations over the HUDF09/XDF allow us to further assess this candidate. These observations show that this candidate, previously only detected at 5.9σ in a single band, clearly corresponds to a real source. It is detected at 5.3σ in the new H ₁₆₀-band data and at 7.8σ in the full 85-orbit H ₁₆₀-band stack. Interestingly, the non-detection of the source (<1σ) in the new F140W observations suggests a higher redshift. Formally, the best-fit redshift of the source utilizing all the WFC3+ACS (and IRAC+K_s -band) observations is 11.8 ± 0.3. However, we consider the z 12 interpretation somewhat unlikely, since the source would either need to be 20× more luminous than expected or show very high-EW Lyα emission (which seems improbable given the extensive neutral gas prevalent early in the reionization epoch). Lower-redshift solutions fail if only continuum models are allowed. Plausible lower-redshift solutions require that the H ₁₆₀-band flux be dominated by line emission such as Hα or [O III] with extreme EWs. The tentative detection of line emission at 1.6 μm in UDFj-39546284 in a companion paper suggests that such emission may have already been found.