Galaxy clusters are the most massive virialized structures in the Universe and are formed through the gravitational accretion of matter over cosmic time¹. The discovery² of an evolved galaxy cluster at redshift z = 2, corresponding to a look-back time of 10.4 billion years, provides an opportunity to study its properties. The galaxy cluster XLSSC 122 was originally detected as a faint, extended X-ray source in the XMM Large Scale Structure survey and was revealed to be coincident with a compact over-density of galaxies² with photometric redshifts of 1.9 ± 0.2. Subsequent observations³ at millimetre wavelengths detected a Sunyaev–Zel’dovich decrement along the line of sight to XLSSC 122, thus confirming the existence of hot intracluster gas, while deep imaging spectroscopy from the European Space Agency’s X-ray Multi-Mirror Mission (XMM-Newton) revealed⁴ an extended, X-ray-bright gaseous atmosphere with a virial temperature of 60 million Kelvin, enriched with metals to the same extent as are local clusters. Here we report optical spectroscopic observations of XLSSC 122 and identify 37 member galaxies at a mean redshift of 1.98, corresponding to a look-back time of 10.4 billion years. We use photometry to determine a mean, dust-free stellar age of 2.98 billion years, indicating that star formation commenced in these galaxies at a mean redshift of 12, when the Universe was only 370 million years old. The full range of inferred formation redshifts, including the effects of dust, covers the interval from 7 to 13. These observations confirm that XLSSC 122 is a remarkably mature galaxy cluster with both evolved stellar populations in the member galaxies and a hot, metal-rich gas composing the intracluster medium.

星系团是宇宙中质量最大的病毒化结构，是通过物质在宇宙时间内的引力吸积形成的¹。在红移z = ²处发现演化星系团 2，对应于 104 亿年的回溯时间，为研究其性质提供了机会。星系团 XLSSC 122 最初在 XMM 大尺度结构调查中被检测为微弱的扩展 X 射线源，并被发现与光度红移为 1.9 ± 0.2的致密超密度星系^2一致。后续观察³在毫米波长处检测到沿着视线到 XLSSC 122 的 Sunyaev-Zel'dovich 衰减，从而证实存在热的星团内气体，而来自欧洲航天局 X 射线多镜任务 (XMM-Newton) 的深度成像光谱透露⁴一种扩展的、X 射线明亮的气态大气，维里温度为 6000 万开尔文，富含金属，其含量与局部星团相同。在这里，我们报告了 XLSSC 122 的光学光谱观测结果，并确定了 37 个平均红移为 1.98 的成员星系，对应于 104 亿年的回溯时间。我们使用光度测量法确定了 29.8 亿年的平均无尘恒星年龄，这表明这些星系中的恒星形成开始于平均红移 12 度，而当时宇宙只有 3.7 亿年的历史。推断的地层红移的全部范围，包括尘埃的影响，覆盖了从 7 到 13 的区间。这些观测证实 XLSSC 122 是一个非常成熟的星系团，成员星系中既有演化的恒星种群，也有一个炽热的、