A Tentative Detection of an Emission Line at 1.6 μm for the z ~ 12 Candidate UDFj-39546284

Brammer, G. B., van Dokkum, P. G., Illingworth, G. D., Bouwens, R. J., Labbe, I., Franx, M., Momcheva, I., and Oesch, P. A.


We present deep WFC3 grism observations of the candidate z ~ 12 galaxy UDFj-39546284 in the Hubble Space Telescope (HST) Ultra Deep Field (UDF), by combining spectroscopic data from the 3D-HST and CANDELS surveys. The total exposure time is 40.5 ks and the spectrum covers 1.10 < λ < 1.65 μm. We search for faint emission lines by cross-correlating the two-dimensional G141 spectrum with the observed H 160 morphology, a technique that is unique to slitless spectroscopy at HST resolution. We find a 2.7σ detection of an emission line at 1.599 μm—just redward of the JH 140 filter—with flux 3.5 ± 1.3 × 10-18 erg s-1 cm-2. Assuming that the line is real, it contributes 110% ± 40% of the observed H 160 flux and has an observed equivalent width >7300 Å. If the line is confirmed, it could be Lyα at z = 12.12. However, a more plausible interpretation, given current results, could be a lower redshift feature such as [O III]λ4959,5007 at z = 2.19. We find two other 3D-HST [O III] emitters within 1000 km s-1 of that redshift in the GOODS-South field. Additional support for this interpretation comes from the discovery of a bright "[O III] blob" with a secure G141 grism redshift of z = 1.605. This object has a strikingly large observed equivalent width of nearly 9000 Å that results in similar "dropout" colors as UDFj-39546284.

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