The chemical enrichment in the early Universe as probed by JWST via direct metallicity measurements at z~8

TL;DR

This study uses JWST to measure metallicities in three galaxies at z~8, providing new insights into early Universe chemical enrichment through direct methods and highlighting calibration challenges.

Contribution

First application of the direct Te method for metallicity at z~8 using JWST, revealing diverse chemical properties and challenging existing calibration relations.

Findings

Metallicities range from extremely metal poor to about one-third solar.

Two galaxies follow the z~2-3 mass-metallicity relation, one is significantly more metal-poor.

The galaxies show different deviations from the Fundamental Metallicity Relation.

Abstract

We analyse the chemical properties of three z~8 galaxies behind the galaxy cluster SMACS J0723.3-7327, observed as part of the Early Release Observations programme of the James Webb Space Telescope (JWST). Exploiting [O III]4363 auroral line detections in NIRSpec spectra, we robustly apply the direct Te method for the very first time at such high redshift, measuring metallicities ranging from extremely metal poor (12+log(O/H)~7) to about one-third solar. We also discuss the excitation properties of these sources, and compare them with local strong-line metallicity calibrations. We find that none of the considered diagnostics match simultaneously the observed relations between metallicity and strong-line ratios for the three sources, implying that a proper re-assessment of the calibrations may be needed at these redshifts. On the mass-metallicity plane, the two galaxies at z~7.6 (log(M*/M_sun) = 8.1, 8.7) have metallicities that are consistent with the extrapolation of the mass-metallicity relation at z~2-3, while the least massive galaxy at z~8.5 (log(M*/M_sun) = 7.8) shows instead a significantly lower metallicity . The three galaxies show different level of offset relative to the Fundamental Metallicity Relation, with two of them (at z~7.6) being marginally consistent, while the z~8.5 source deviating significantly, being probably far from the smooth equilibrium between gas flows, star formation and metal enrichment in place at later epochs.