2013ApJ...765L...2B

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 H₁₆₀morphology, 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 JH₁₄₀filter–with flux 3.5±1.3x10^–18 erg/s/cm2. Assuming that the line is real, it contributes 110%±40% of the observed H₁₆₀ 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 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.