Within the established framework of structure formation, galaxies start as systems of low stellar mass and gradually grow into far more massive galaxies. The existence of massive galaxies in the first billion years of the Universe, as suggested by recent observations, seems to challenge this model, as such galaxies would require highly efficient conversion of baryons into stars. An even greater challenge in this epoch is the existence of massive galaxies that have already ceased forming stars. However, robust detections of early massive quiescent galaxies have been challenging due to the coarse wavelength sampling of photometric surveys. Here we report the spectroscopic confirmation with the James Webb Space Telescope of the quiescent galaxy RUBIES-EGS-QG-1 at redshift z = 4.90, 1.2 billion years after the Big Bang. Deep stellar absorption features in the spectrum reveal that the stellar mass of the galaxy of 10¹¹ M_⊙ formed in a short 200 Myr burst of star formation, after which star formation activity dropped rapidly and persistently. According to current galaxy formation models, systems with such rapid stellar mass growth and early quenching are too rare to plausibly occur in the small area probed spectroscopically with JWST. Instead, the discovery of RUBIES-EGS-QG-1 implies that early massive quiescent galaxies can be quenched earlier or exhaust gas available for star formation more efficiently than assumed at present.

在既定的结构形成框架内，星系从低质量的星系开始，逐渐发展成质量大得多的星系。正如最近的观测所表明的那样，在宇宙的前十亿年中存在大质量星系，这似乎对这一模型提出了挑战，因为这样的星系需要重子高效地转化为恒星。在这个时代，一个更大的挑战是存在已经停止形成恒星的大质量星系。然而，由于光度巡天测量的粗波长采样，对早期大质量静止星系的稳健探测一直具有挑战性。在这里，我们报告了詹姆斯·韦伯太空望远镜在大爆炸后 12 亿年红移 z = 4.90 处对静止星系 RUBIES-EGS-QG-1 的光谱确认。光谱中的深层恒星吸收特征表明，10¹¹M_⊙ 星系的恒星质量是在短暂的 200 Myr 恒星形成爆发中形成的，之后恒星形成活动迅速而持续地下降。根据目前的星系形成模型，具有如此快速恒星质量增长和早期猝灭的系统太罕见了，不可能出现在用 JWST 光谱探测的小区域。相反，RUBIES-EGS-QG-1 的发现意味着早期的大质量静止星系可以比目前假设的更早地被淬灭，或者可以更有效地为恒星形成提供废气。