First look with JWST spectroscopy: $z \sim 8$ galaxies resemble local analogues

Understand

Deep images and near-IR spectra of galaxies in the field of the lensing cluster SMACS J0723.3-7327 were recently taken in the Early Release Observations program of JWST.

• Among these, two NIRSpec spectra of galaxies at $z=7.7$ and one at $z=8.5$ were obtained, revealing for the first time rest-frame optical emission line spectra of galaxies in the epoch of reionization, including the detection of the important[OIII]4363 auroral line (see JWST PR 2022-035).
• We present an analysis of the emission line properties of these galaxies, finding that these galaxies have a high excitation (as indicated by high ratios of [OIII]/[OII], [NeIII]/[OII]), strong [OIII]4363/H$\gamma$, high equivalent widths, and other properties which are typical of low-metallicity star-forming galaxies.
• Using the direct method we determine oxygen abundances of $12+\log(O/H)=7.9$ in two $z=7.7$ galaxies, and a lower metallicity of $12+\log(O/H)\approx 7.4-7.5$ in the $z=8.5$ galaxy using different strong line methods.