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

TL;DR

The study uses JWST/NIRSpec to obtain direct electron-temperature-based metallicities for three z~8 galaxies behind SMACS J0723, enabled by detection of [OIII]4363. It demonstrates that Te-based abundances span from extremely metal-poor to roughly one-third solar and reveals that local strong-line calibrations do not consistently reproduce the observed line ratios at these redshifts. The two more massive z~7.6 systems lie near the z~2–3 mass–metallicity relation, while the lowest-mass z~8.5 system is significantly under-enriched, suggesting rapid enrichment or evolving MZR slopes at low masses. Two of the three galaxies are marginally consistent with the Fundamental Metallicity Relation, whereas the z~8.5 object is far from equilibrium, indicating that early galaxies experience non-steady gas flows and metal enrichment. Comparisons with cosmological simulations show partial agreement, highlighting the need for larger high-z samples and recalibration of metallicity diagnostics in the early Universe.

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.

Figures (8)

• Figure 1: JWST/NIRSpec spectra of the three sources analysed in this work. Several Hydrogen, Helium and metals emission lines are detected; the best-fit models are highlighted in blue. The data and best-fit continuum are traced by the solid grey and red lines, respectively; the dots are the residuals. The bottom panels show a zoom-in on the spectral region of three groups of lines; from left to right they are: the [Oii]$\lambda3727,29$ doublet, H$\updelta$, H$\upgamma$ and [Oiii]4363, and H$\upbeta$ and [Oiii]$\lambda5007$. Grey regions have been masked due to artefacts (either in the spectrum or in the noise) or sigma clipping; the wide grey region at 4450-4750 Å in the top panel of ID 4590 falls in the detector gap.
• Figure 1: (continued)
• Figure 2: Flux ratios between different Balmer lines and the H$\beta$ as observed in all galaxies' spectra. The theoretical values expected from atomic physics (assuming the case B recombination at T$=1.5\times10^{4}$K and N$_{\rm e}$ = 300 cm${-3}$) are marked by the blue lines. All observed ratios are consistent with little dust attenuation. Grey shaded areas mark regions with unreliable ratios: non-physical H$\zeta$/H$\beta$ ratio might be driven by H$\zeta$ flux being contaminated by He I emission, whereas residuals in the background subtraction may affect the H$\delta$ emission in ID$4590$ and ID$10612$.
• Figure 3: [Oiii]/H$\beta$ versus [Oii]/H$\beta$ diagram (left-hand panel) and [Oiii]/[Oii] versus R$_{23}$ diagram (right-hand panel) for local SDSS galaxies (grey contours marking the 70% and 90% of the distribution), galaxies at $z\sim$1--3 from the literature, and the JWST sample at $z\sim8$.
• Figure 4: [Oiii]/H$\beta$ vs [Oii]/H$\beta$ diagram illustrating the photoionization models for low metallicity PopII, PopIII and AGNs from Nakajima_Maiolino22. Different symbols mark different population of objects (as explained in the legend), while different colours belong to different values of the gas-phase metallicity. The line ratios observed in our $z\sim$ 8 galaxies are marked with red symbols.