The Detection of Ionized Carbon Emission at z ∼ 8*

We present deep Keck/MOSFIRE H-band spectroscopic observations covering the [C iii],C iii]λλ1907, 1909 doublet for three z ∼ 8 galaxy candidates in the AEGIS field. Along with nondetections in two galaxies, we obtain one of the highest-redshift detections to date of [C iii]λ1907 for the galaxy AEGIS-33376, finding zspec = 7.945 ± 0.001. We measure a [C iii]λ1907 flux of 2.24 ± 0.71 × 10−18erg s−1cm−2, corresponding to a rest-frame equivalent width of 20.3 ± 6.5 Å for the single line. Given the not very constraining upper limit for C iii]λ1909 based on strong sky-line contamination, we assume a [C iii]λ1907/C iii]λ1909 doublet ratio of 1.5 and infer a total [C iii][C iii]λλ1907, 1909 equivalent width of 33.7 ± 10.8 Å. We repeat the same reductions and analysis on multiple subsets of our raw data divided on the basis of time and observing conditions, verifying that the [C iii]λ1907 emission is present for AEGIS-33376 throughout our observations. We also confirm that the significance of the [C iii]λ1907 detection in different subsets of our data tracks that of brighter emission features detected on the same multi-slit mask. These multiple tests suggest that the observed emission line is real and associated with the z ∼ 8 target. The strong observed [C iii],C iii]λλ1907, 1909 in AEGIS-33376 likely indicates ISM conditions of low metallicity, high ionization parameter, and a hard ionizing spectrum, although AGN contributions are possible. This single detection represents a sizable increase in the current sample [C iii],C iii]λλ1907, 1909 detections at z > 7, while JWST will provide the first statistical samples of such measurements at these redshifts.