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 similar to 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 < lambda < 1.65 mu m. 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 2.7 sigma detection of an emission line at 1.599 mu m-just redward of the JH(140) filter-with flux 3.5 +/- 1.3 x 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 angstrom. If the line is confirmed, it could be Ly alpha at z = 12.12. However, a more plausible interpretation, given current results, could be a lower redshift feature such as [O III]lambda 4959,5007 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 angstrom that results in similar dropout colors as UDFj-39546284.