A Tentative Detection of an Emission Line at 1.6 {\mu}m for the z~12 Candidate UDFj-39546284

TL;DR

This study reports a tentative detection of an emission line in a high-redshift galaxy candidate, exploring whether it indicates a z~12 galaxy or a lower-redshift interloper, with implications for early universe observations.

Contribution

The paper introduces a novel spectroscopic analysis technique for slitless HST data and presents the first tentative emission line detection in a z~12 galaxy candidate.

Findings

Detected a 2.7σ emission line at 1.599 μm in the candidate galaxy.

The line could be Ly-α at z=12.12 or [O III] at z=2.19, with current evidence favoring the latter.

Identified nearby [O III] emitters and a z=1.605 [O III] blob supporting a lower-redshift interpretation.

Abstract

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}. 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 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 {\AA} that results in similar "dropout" colors as UDFj-39546284.