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

Abstract

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 {\lt} {$λ$} {\lt} 1.65 {$μ$}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{$σ$} detection of an emission line at 1.599 {$μ$}m{\mdash}just redward of the JH $_{140}$ filter{\mdash}with flux 3.5 {\plusmn} 1.3 {\times} 10$^{-18}$ erg s$^{-1}$ cm$^{-2}$. Assuming that the line is real, it contributes 110\% {\plusmn} 40\% of the observed H $_{160}$ flux and has an observed equivalent width {\gt}7300 Å. If the line is confirmed, it could be Ly{$α$} at z = 12.12. However, a more plausible interpretation, given current results, could be a lower redshift feature such as [O III]{$λ$}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 Å that results in similar ''dropout'' colors as UDFj-39546284. Based on observations made with the NASA/ESA Hubble Space Telescope, programs GO-12099, 12177, and 12547, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.