A pilot survey for C III] emission in the reionization era: gravitationally lensed z ∼ 7–8 galaxies in the frontier fields cluster Abell 2744

We report results of a search for C iii] $\lambda \lambda$1907, 1909 Å emission using Keck's MOSFIRE spectrograph in a sample of 7 $z_{phot}\sim 7-8$ candidates ($H\sim 27$) lensed by the Hubble Frontier Field cluster Abell 2744. Earlier work has suggested the promise of using the C iii] doublet for redshift confirmation of galaxies in the reionization era given that Lyα (λ1216 Å) is likely attenuated by the neutral intergalactic medium. The primary challenge of this approach is the feasibility of locating C iii] emission without advanced knowledge of the spectroscopic redshift. With an integration time of 5 hr in the H band, we reach a $5\sigma$ median flux limit (in between the skylines) of $1.5\times {10}^{-18}$ ergs cm⁻² s⁻¹ but no convincing C iii] emission was found. We also incorporate preliminary measurements from two other CLASH/HFF clusters in which, similarly, no line was detected, but these were observed to lesser depth. Using the known distribution of OH emission and the photometric redshift likelihood distribution of each lensed candidate, we present statistical upper limits on the mean total C iii] rest-frame equivalent width (EW) for our $z\simeq 7-8$ sample. For a signal-to-noise ratio of 5, we estimate that the typical C iii] doublet rest-frame EW is, with 95% confidence, $<26\pm 5$ Å. Although consistent with the strength of earlier detections in brighter objects at $z\simeq 6-7$, our study illustrates the necessity of studying more luminous or strongly lensed examples prior to the launch of the James Webb Space Telescope.