Gabriel B. Brammer, Pieter G. van Dokkum, Garth D. Illingworth, Rychard J. Bouwens, Ivo Labbe, Marijn Franx, Ivelina Momcheva, Pascal A. Oesch
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 < {\lambda} < 1.65{\mu}m. 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 2.7{\sigma} detection of an emission line at 1.599 {\mu}m---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-{\alpha} 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-{\alpha} at z=12.12, but such an object would be very unusual. A possible alternative identification of the line is [O III]{\lambda}4959+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.
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http://arxiv.org/abs/1301.0317
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