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

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]λ4363 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 ($\rm log(M_*/M_{\odot }) = 8.1, 8.7$) 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 ($\rm log(M_*/M_{\odot }) = 7.8$) 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.

A First Look at the Abundance Pattern—O/H, C/O, and Ne/O—in z > 7 Galaxies with JWST/NIRSpec

“…Such results are consistent with respect to those derived in this work (see Table 2). However, for the o008 visit of s04590, we reported a value 0.19 dex higher than that obtained using the measurements of Curti et al (2023) but consistent within the errors. Despite the different approaches used to derive reddening and T e [O III], both results are in agreement with the Ne/O abundance ratios estimated in local star-forming regions at low metallicity (12 + log(O/H) < 8.0) and with respect to the solar abundance as reported in local star-forming regions (see, e.g., Dors et al 2013).…”

“… 0.3 Z e ). To explore the impact of the highly uncertain reddening correction, we also calculated the metallicity using uncorrected line fluxes (E(B − V ) = 0), finding that the O/H abundances decrease by 0.12 dex, on average, which is within the uncertainties estimated in this study (see also Curti et al 2023).…”

“…Following Trump et al (2022), we adopt large (0. Curti et al (2023), andRhoads et al (2022). Note that the results of Schaerer et al (2022) and Rhoads et al (2022) are obtained using the original JWST Science Calibration Pipeline spectra, while Trump et al (2022) and Curti et al (2023) used their own postprocessing reduction.…”

“…In addition, to investigate the impact of the reddening in determining the Ne/O ratio, we derived the Ne/O abundance ratio using the intensities of [O II], [O III], and [Ne III] and T e [O III] reported in Curti et al (2023). The intensities were corrected by reddening using the A v values also reported in Curti et al (2023). We calculated values of the Ne/O ratios of −0.71 ± 0.25, −0.67 ± 0.12, and −0.58 ± 0.17 for s04590, s06355, and s10612.…”

“…Electron temperatures were calculated from the [O III] λ4363/λ5007 line ratio and are given in units of 10 4 K. The fifth through seventh rows list the individual values and comparative differences for the derived abundances: 12+log(O/H), log(C/O), and log(Ne/O). a In italic the spectrum affected by shutter closures (seeCurti et al 2023).…”