Photometric Constraints on the Redshift of z ~ 10 Candidate UDFj-39546284 from Deeper WFC3/IR+ACS+IRAC Observations over the HUDF

Bouwens, R. J., Oesch, P. A., Illingworth, G. D., Labbé, I., van Dokkum, P. G., Brammer, G., Magee, D., Spitler, L. R., Franx, M., Smit, R., Trenti, M., Gonzalez, V., and Carollo, C. M.

Abstract

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+Ks -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.

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