Dating the ages and weighting the stellar populations in galaxies are essential steps when studying galaxy formation through cosmic times. Evolutionary population synthesis models with different input physics are used for this purpose. Moreover, the contribution from the thermally pulsing asymptotic giant branch (TP-AGB) stellar phase, which peaks for intermediate-age 0.6–2 Gyr systems, has been debated for decades. Here we report the detection of strong cool-star signatures in the rest-frame near-infrared spectra of three young (~1 Gyr), massive (~10¹⁰ M_⊙) quiescent galaxies at large look-back time, z = 1–2, using JWST/NIRSpec. The coexistence of oxygen- and carbon-type absorption features, spectral edges and features from rare species, such as vanadium and possibly zirconium, reveal a strong contribution from TP-AGB stars. Population synthesis models with a significant TP-AGB contribution reproduce the observations better than those with a weak TP-AGB, which are commonly used. These findings call for revisions of published stellar population fitting results, as they point to populations with lower masses and younger ages and have further implications for cosmic dust production and chemical enrichment. New generations of improved models are needed, informed by these and future observations.

在研究宇宙时期的星系形成时，确定星系的年龄并确定恒星种群的加权是必不可少的步骤。为此，使用了具有不同输入物理场的进化种群合成模型。此外，热脉冲渐近巨分支 （TP-AGB） 恒星相位的贡献，在中龄 0.6-2 Gyr 系统中达到峰值，几十年来一直存在争议。在这里，我们报告了使用 JWST/NIRSpec 在大回溯时间 z = 1-2 的三个年轻 （~1 Gyr）、大质量 （~10¹⁰M_⊙） 静止星系的静止帧近红外光谱中检测到强烈的冷星特征。氧型和碳型吸收特征、光谱边缘和稀有物种（如钒和可能的锆）的特征共存，揭示了 TP-AGB 星的强大贡献。具有显著 TP-AGB 贡献的群体合成模型比常用的具有弱 TP-AGB 的模型更好地再现了观察结果。这些发现需要对已发表的恒星种群拟合结果进行修订，因为它们指向质量较低且年龄较小的种群，并且对宇宙尘埃的产生和化学富集具有进一步的影响。需要新一代的改进模型，以这些和未来的观察结果为依据。