The mass–metallicity relation provides crucial insights into the baryon cycle in galaxies and strong constraints on galaxy formation models. We use JWST NIRSpec observations from the UNCOVER program to measure the gas-phase metallicity in a sample of eight galaxies during the epoch of reionization at z = 6–8. Thanks to the strong lensing of the galaxy cluster Abell 2744, we are able to probe extremely low stellar masses between 10⁶ and 10⁸ M _⊙. Using strong-line diagnostics and the most recent JWST calibrations, we derive extremely low oxygen abundances in the range of 12 + log(O/H) = 6.7–7.8. By combining this sample with more massive galaxies at similar redshifts, we derive a best-fit relation of 12 + log(O/H) = −0.076−0.03+0.03×(log(M⋆))2+1.61−0.52+0.52 × log(M⋆)−0.26−0.10+0.10 , which becomes steeper than determinations at z ∼ 3–6 toward low-mass galaxies. Our results show a clear redshift evolution in the overall normalization of the relation, galaxies at higher redshift having significantly lower metallicities at a given mass. A comparison with theoretical models provides important constraints on which physical processes, such as metal mixing, star formation or feedback recipes, are important in reproducing the observations. Additionally, these galaxies exhibit star formation rates that are higher by a factor of a few to tens compared to extrapolated relations at similar redshifts or theoretical predictions of main-sequence galaxies, pointing to a recent burst of star formation. All these observations are indicative of the highly stochastic star formation and interstellar medium enrichment expected in these low-mass systems, suggesting that feedback mechanisms in high-z dwarf galaxies might be different from those in place at higher masses.

中文翻译：

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质量-金属丰度关系在 z ∼ 7 处的极低质量端


质量-金属丰度关系为星系中的重子循环和星系形成模型的强大约束提供了重要的见解。我们使用 UNCOVER 程序中的 JWST NIRSpec 观测来测量 z = 6-8 再电离时期八个星系样品中的气相金属丰度。由于星系团 Abell 2744 的强透镜效应，我们能够探测到 10⁶ 到 10⁸M_⊙ 之间的极低质量恒星。使用强线诊断和最新的 JWST 校准，我们得出了 12 + log（O/H） = 6.7–7.8 范围内的极低氧丰度。通过将这个样本与具有相似红移的更大质量星系相结合，我们得出了 12 + log（O/H） = −0.076−0.03+0.03×（log（M⋆））2+1.61−0.52+0.52 × log（M⋆）−0.26−0.10+0.10 的最佳拟合关系，它比在 z ∼ 3-6 处对低质量星系的测定更陡峭。我们的结果表明，在关系的整体归一化中，红移演化明显，在给定质量下，较高红移的星系具有明显较低的金属丰度。与理论模型的比较为哪些物理过程（如金属混合、恒星形成或反馈配方）在重现观测结果中很重要提供了重要的约束。此外，与相似红移的外推关系或主序星系的理论预测相比，这些星系表现出的恒星形成速率高出几到几十倍，这表明最近恒星的形成爆发。 所有这些观测都表明，在这些低质量系统中，预期的高度随机恒星形成和星际介质富集，这表明高 z 矮星系中的反馈机制可能与高质量星系中的反馈机制不同。