Protoclusters, the progenitors of the most massive structures in the Universe, have been identified at redshifts of up to 6.6 (refs. ^1,2,3,4,5,6). Besides exploring early structure formation, searching for protoclusters at even higher redshifts is particularly useful to probe the reionization. Here we report the discovery of the protocluster LAGER-z7OD1 at a redshift of 6.93, when the Universe was only 770 million years old and could be experiencing rapid evolution of the neutral hydrogen fraction in the intergalactic medium^7,8. The protocluster is identified by an overdensity of 6 times the average galaxy density, and with 21 narrowband selected Lyman-α galaxies, among which 16 have been spectroscopically confirmed. At redshifts similar to or above this record, smaller protogroups with fewer members have been reported^9,10. LAGER-z7OD1 shows an elongated shape and consists of two subprotoclusters, which would have merged into one massive cluster with a present-day mass of 3.7 × 10¹⁵ solar masses. The total volume of the ionized bubbles generated by its member galaxies is found to be comparable to the volume of the protocluster itself, indicating that we are witnessing the merging of the individual bubbles and that the intergalactic medium within the protocluster is almost fully ionized. LAGER-z7OD1 thus provides a unique natural laboratory to investigate the reionization process.

原星团是宇宙中最大规模结构的祖先，已被确定为红移高达 6.6（参考文献 ^{1、2、3、4、5、6}）。除了探索早期结构形成之外，寻找更高红移的原星团对于探测再电离特别有用。在这里，我们报告了原星团 LAGER- z 7OD1 在红移 6.93 时的发现，当时宇宙只有 7.7 亿年的历史，并且可能正在经历星系间介质中中性氢部分的快速演化^7,8. 原星团的超密度是平均星系密度的 6 倍，有 21 个窄带选定的莱曼-α 星系，其中 16 个已通过光谱确认。在与该记录相似或更高的红移处，据报道成员较少的较小的原始群^9,10。LAGER- z 7OD1 呈细长形状，由两个亚原星团组成，它们将合并成一个巨大的星团，目前质量为 3.7 × 10 ¹⁵太阳质量。发现其成员星系产生的电离气泡的总体积与原星系团本身的体积相当，这表明我们正在目睹单个气泡的合并，并且原星系团内的星系际介质几乎完全电离。因此， LAGER- z 7OD1 提供了一个独特的天然实验室来研究再电离过程。