Recent James Webb Space Telescope (JWST) observations have revealed an unexpected abundance of massive-galaxy candidates in the early Universe, extending further in redshift and to lower luminosity than what had previously been found by submillimetre surveys^1,2,3,4,5,6. These JWST candidates have been interpreted as challenging the Λ cold dark-matter cosmology (where Λ is the cosmological constant)^7,8,9, but, so far, these studies have mostly relied on only rest-frame ultraviolet data and have lacked spectroscopic confirmation of their redshifts^{10,11,12,13,14,15,16}. Here we report a systematic study of 36 massive dust-obscured galaxies with spectroscopic redshifts between 5 and 9 from the JWST FRESCO survey. We find no tension with the Λ cold dark-matter model in our sample. However, three ultra-massive galaxies (logM_★/M_⊙ ≳ 11.0, where M_★ is the stellar mass and M_⊙ is the mass of the Sun) require an exceptional fraction of 50 per cent of baryons converted into stars—two to three times higher than the most efficient galaxies at later epochs. The contribution from an active galactic nucleus is unlikely because of their extended emission. Ultra-massive galaxies account for as much as 17 per cent of the total cosmic star-formation-rate density¹⁷ at redshifts between about five and six.

詹姆斯·韦伯太空望远镜 （JWST） 最近的观测揭示了早期宇宙中出乎意料的大质量星系候选者，红移进一步延伸，光度比以前亚毫米级巡天发现的光度更低^1,2,3,4,5,6。这些 JWST 候选者被解释为挑战 Λ 冷暗物质宇宙学（其中 Λ 是宇宙学常数）^7,8,9，但到目前为止，这些研究主要仅依赖于静止框架紫外线数据，缺乏对它们的红移^{10、11、12、13、14、15、16} 的光谱确认。在这里，我们报告了对 JWST FRESCO 巡天中 36 个大质量尘埃遮蔽星系的系统研究，这些星系的光谱红移在 5 到 9 之间。我们在样本中发现 Λ 冷暗物质模型没有张力。然而，三个超大质量星系（logM_★/M_⊙≳ 11.0，其中M_★是恒星质量，M_⊙是太阳的质量）需要50%的重子转化为恒星——比后来最高效的星系高出两到三倍。由于活跃的星系核的发射时间延长，因此不太可能做出贡献。超大质量星系占宇宙恒星形成速率密度的^{17% 之多 17}，红移约为 5 到 6 之间。