The mass assembly and chemical enrichment of the first galaxies provide key insights into their star formation histories and the earliest stellar populations at cosmic dawn. Here we compile and utilise new, high-quality spectroscopic, łambdałambda 4959, 5007 doublet and resolve the auroral, łambda 4363 line for the 11 galaxies in our sample (z=9. 3-10. 0) to obtain direct Tₑ-based metallicity measurements. We find that the interstellar medium (ISM) of all galaxies shows high ionisation fields and electron temperatures, with derived metallicities in the range 12+łog Prism observations from the archive. In particular, we extend the wavelength coverage beyond the standard pipeline cut-off (5. 3μm) up to 5. 5, μm, which enables for the first time a detailed examination of the rest-frame optical emission-line properties for galaxies at z≈ 10. Crucially, the improved calibration allows us to detect Hβ and the iii ̊m (O/H) = 7. 1 - 8. 3 (3--50% solar), consistent with previous strong-line diagnostics based on) and with evidence of bursty star formation on 10, Myr versus 100, Myr timescales (log_ () ≈ 0. 7). Combining the rest-frame optical line analysis and detailed UV to optical spectro-photometric modelling, we determine the mass-metallicity relation (MZR) and the fundamental metallicity relation (FMR) of the sample, pushing the previous redshift frontier of these measurements to z=10. These results, together with literature measurements, point to a gradually decreasing MZR at higher redshifts, with a break in the FMR at z≈ 3, decreasing to metallicities ≈ 3 data at high redshifts. We derive an empirical relation for M_̊m UV and 12+log (O/H) at z≈ 10, useful for future higher-redshift studies, and show that the sample galaxies are `typical' star-forming galaxies though with relatively high specific star formation rates (median sSFR = SFR_̊m Hβ/M_⋆ = 38, Gyr -1 10 ̊m SFR_ 10 /SFR_ 100 lower at z=10 than observed in galaxies during the majority of cosmic time at z=0-3, likely caused by massive pristine gas inflows diluting the observed metal abundances during early galaxy assembly at cosmic dawn.
Pollock et al. (Fri,) studied this question.