[Paper Review] First look with JWST spectroscopy: $z \sim 8$ galaxies resemble local analogues

D. Schaerer, R. Marques-Chaves+6 more|arXiv (Cornell University)|Jul 20, 2022

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study presents the first rest-frame optical spectroscopy of galaxies at $z \sim 8$ using JWST/NIRSpec, revealing that their emission line properties—high excitation, strong [O iii] $\lambda$4363, low metallicity ($12+\log({\rm O/H}) \approx 7.4-8.0$), and high equivalent widths—closely resemble those of rare, low-metallicity local star-forming galaxies. The findings suggest that early high-redshift galaxies are chemically and physically similar to local analogues, enabling direct comparison with well-studied low-$z$ systems.

ABSTRACT

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$γ$, 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. More accurate metallicity determinations will require better data. With stellar masses estimated from SED fits, we find that the three galaxies lie close to or below the $z \sim 2$ mass-metallicity relation. Overall, these first galaxy spectra at $z \sim 8$ show a strong resemblance of the emission lines properties of galaxies in the epoch of reionization with those of relatively rare local analogues previously studied from the SDSS. Clearly, the first JWST observations demonstrate already the incredible power of spectroscopy to reveal properties of galaxies in the early Universe.

Motivation & Objective

• To analyze the rest-frame optical emission line spectra of $z \sim 8$ galaxies observed in the JWST Early Release Observations.
• To determine the metallicity of high-redshift galaxies using the direct method and strong-line indicators.
• To compare the physical properties of $z \sim 8$ galaxies with those of local low-metallicity star-forming galaxies, or 'local analogues'.
• To assess the implications of these findings for galaxy evolution and the epoch of reionization.
• To evaluate the feasibility of using local analogues as proxies for understanding early high-redshift galaxies.

Proposed method

• Acquisition of deep, continuous near-IR spectra (1.8–5.2 $\mu$m) using JWST/NIRSpec with a spectral resolution $R \approx 1000$.
• Combination and calibration of 1D spectra from two observing pointings (s007 and s008) using the JWST Science Calibration Pipeline (v1.5.3).
• Flux correction and artifact masking based on inspection of 2D spectra to ensure accurate line flux measurements.
• Application of the direct method to determine oxygen abundance using the detected [O iii] $\lambda$4363 auroral line.
• Use of strong-line methods (e.g., O3N2, N2) to cross-validate metallicity estimates.
• Stellar mass estimation via SED fitting to compare galaxies with the $z \sim 2$ mass-metallicity relation.

Experimental results

Research questions

• RQ1 Do galaxies at $z \sim 8$ exhibit emission line properties similar to those of low-metallicity local star-forming galaxies?
• RQ2 What is the metallicity of $z \sim 8$ galaxies, and how does it compare to the $z \sim 2$ mass-metallicity relation?
• RQ3 How do the line ratios (e.g., O32, Ne3O2) and equivalent widths of emission lines in $z \sim 8$ galaxies compare to those of local analogues?
• RQ4 What is the ionizing photon production efficiency ($\xi_{\rm ion}$) of these high-redshift galaxies?
• RQ5 To what extent can local low-metallicity galaxies serve as physical analogues for galaxies in the epoch of reionization?

Key findings

• The [O iii] $\lambda$4363 auroral line is detected with significance $3.9-5.7\sigma$ in all three $z \sim 8$ galaxies, enabling direct metallicity determination via the direct method.
• Oxygen abundances are measured at $12+\log({\rm O/H}) \approx 7.9$ in two $z=7.7$ galaxies and $7.4-7.5$ in the $z=8.5$ galaxy, corresponding to $\sim 5-20\%$ solar metallicity.
• The galaxies exhibit high excitation, with O32 ratios of 6–11 and Ne3O2 ratios of 0.4–0.7, typical of low-metallicity star-forming systems.
• Equivalent widths of [O iii] $\lambda$5007 reach up to $\sim 700$ Å, indicating strong, hard ionizing radiation fields.
• Ionizing photon production efficiency is estimated at $\log(\xi_{\rm ion}) \approx 25.2-25.5$ erg$^{-1}$ Hz, consistent with efficient ionizing output.
• Stellar masses place the galaxies close to or below the $z \sim 2$ mass-metallicity relation, suggesting possible evolution or low-metallicity bias.