First look with JWST spectroscopy: Resemblance among z ∼ 8 galaxies and local analogs

Deep images and near-IR spectra of galaxies in the field of the lensing cluster SMACS J0723.3-7327 were recently taken as part of the Early Release Observations (EROs) program of the James Webb Space Telescope (JWST). Among these, two NIRSpec spectra of galaxies, at z = 7.7 and at z = 8.5, were obtained, revealing, for the first time, the rest-frame optical emission line spectra of galaxies in the epoch of reionization, including the detection of the important [O III]lambda 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 [O III]lambda 5007/[O II] lambda 3727, [Ne III] lambda 3869/[O II] lambda 3727), strong [O III]lambda 4363, high equivalent widths, and other properties typical of low-metallicity star-forming galaxies. Using the direct method, we determined oxygen abundances of 12 + log(O/H)approximate to 7.9 in two z = 7.7 galaxies and a lower metallicity of 12 + log(O/H)approximate to 7.4 - 7.5 (similar to 5% solar) in the z = 8.5 galaxy using different strong line methods. More accurate metallicity determinations will require better data. With stellar masses estimated from spectral energy distribution (SED) fits, we find that the three galaxies lie close to or below the z similar to 2 mass-metallicity relation. Overall, these first galaxy spectra at z similar to 8 show a strong resemblance in their the emission line properties of galaxies in the epoch of reionization with those of relatively rare local analogs previously studied with the SDSS. Clearly, the first JWST observations demonstrate already the incredible power of spectroscopy to reveal the properties of galaxies in the early Universe.

Automated summary

Deep images and near-IR spectra of galaxies in the field of the lensing cluster SMACS J0723.3-7327 were recently observed, revealing the rest-frame optical emission line spectra of galaxies in the epoch of reionization for the first time. Analysis of the emission line properties showed high excitation, low metallicity, and similarity to previous studies of local analogs. These findings demonstrate the powerful capabilities of spectroscopy in understanding early Universe galaxies.