The Detection of Ionized Carbon Emission at z~8

TL;DR

This paper reports one of the highest-redshift detections of ionized carbon emission at z~8, providing insights into early galaxy ISM conditions and demonstrating the feasibility of such measurements with current ground-based telescopes.

Contribution

First detection of [CIII] emission at z~8 using Keck/MOSFIRE, expanding the sample of high-redshift galaxy emission line measurements and informing early universe ISM properties.

Findings

Detected [CIII]$ ext{λ}1907$ at z=7.945 with significant confidence.

Indicates low metallicity and high ionization in early galaxy ISM.

Supports future JWST studies of high-redshift galaxy spectra.

Abstract

We present deep Keck/MOSFIRE $H$-band spectroscopic observations covering the [CIII],CIII]$\lambda\lambda1907,1909$ doublet for three $z\sim8$ galaxy candidates in the AEGIS field. Along with non-detections in two galaxies, we obtain one of the highest-redshift detections to-date of [CIII]$\lambda 1907$ for the galaxy AEGIS-33376, finding $z_{\rm spec}=7.945\pm0.001$. We measure a [CIII]$\lambda$1907 flux of $2.24\pm0.71\times10^{-18} \mbox{ erg}\mbox{ s}^{-1} \mbox{ cm}^{-2}$, corresponding to a rest-frame equivalent width of $20.3\pm6.5 \unicode{x212B}$ for the single line. Given the not very constraining upper limit for CIII]$\lambda 1909$ based on strong sky-line contamination, we assume a [CIII]$\lambda$1907/CIII]$\lambda 1909$ doublet ratio of 1.5 and infer a total [CIII],CIII]$\lambda\lambda1907,1909$ equivalent width of $33.7\pm 10.8 \unicode{x212B}$. 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 [CIII]$\lambda 1907$ emission is present for AEGIS-33376 throughout our observations. We also confirm that the significance of the [CIII]$\lambda 1907$ 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 $z\sim 8$ target. The strong observed [CIII],CIII]$\lambda\lambda1907,1909$ 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 [CIII],CIII]$\lambda\lambda1907,1909$ detections at $z>7$, while $\textit{JWST}$ will provide the first statistical samples of such measurements at these redshifts.