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

We present deep WFC3 grism observations of the candidate z~12 galaxy UDFj-39546284 in the 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 < 1.65μm. We search for faint emission lines by cross-correlating the 2D G141 spectrum with the observed H160 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 JH140 filter---with flux 3.5 +/- 1.3x10-18 erg/s/cm2. Assuming the line is real, it contributes 110 +/- 40% of the observed H160 flux and has an observed equivalent width > 7300 \AA. A companion paper argues that without significant contribution of Ly-α to the H160 flux, UDFj-39546284 is >20 times more luminous than expected for a comparably-prevalent z~12 galaxy. The observed line could be Ly-α at z=12.12, but such an object would be very unusual. A possible alternative identification of the line is [O III]λ4959+5007 redshifted to z = 2.19 +/- 0.01. 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=1.6. This object has a strikingly large observed equivalent width of 11000 \AA that results in similar "dropout" colors as UDFj-39546284.