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 z spec = 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.