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 medium7,8. 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 reported9,10. 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.
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The candidate selection is based on the following images in COSMOS field: DECam-NB964 (NOIRLab Prop. ID: 2016A-0386, 2017B-0330; CNTAC Prop. ID: 2016A-0610), HSC SSP programme and HSC-NB973 (Prop. ID: S16B-001I), which are available at http://archive1.dm.noao.edu/, https://hsc-release.mtk.nao.ac.jp/doc/ and https://hsc-release.mtk.nao.ac.jp/doc/index.php/chorus/, respectively. The spectroscopic datasets and the datasets generated or analysed during this study are available from the corresponding authors upon reasonable request. The LAE catalogue for LAGER-z7OD1 used in this study is available in the Supplementary Information.
Codes used in this study are not publicly released yet but are available from the corresponding authors on reasonable request.
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We thank Z.-Y. Cai, Z. Cai and L. Jiang for informative discussions. The work is supported by the National Science Foundation of China (grant numbers 11421303, 11890693, 11773051 and 12022303), the CAS Frontier Science Key Research Program (QYZDJ-SSW-SLH006) and the CAS Pioneer Hundred Talents Program. US investigators on this work have been suppported by the US National Science Foundation through NSF grant AST-1518057 and by NASA through WFIRST Science Investigation Team contract NNG16PJ33C. This project used the data obtained with the Dark Energy Camera (DECam), which was constructed by the Dark Energy Survey (DES), and public archival data from the DES. This research is based on observations at Cerro Tololo Inter-American Observatory at NSF’s NOIRLab (NOIRLab Prop. ID: 2016A-0386, 2017B-0330, Principal Investigator: S. Malhotra; CNTAC Prop. ID: 2016A-0610, Principal Investigator: L. Infante), which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. This work includes data collected at the 6.5 m Magellan Telescope located at Las Campanas Observatory, Chile. We thank the scientists and telescope operators at Magellan Telescope for their help. This paper makes use of software developed for the Large Synoptic Survey Telescope. We thank the LSST Project for making their code available as free software at http://dm.lsst.org. This work is based (in part) on data collected at the Subaru Telescope and retrieved from the HSC data archive system, which is operated by the Subaru Telescope and Astronomy Data Center at the National Astronomical Observatory of Japan. Facilities: Magellan Baade (IMACS), Magellan Clay (LDSS3), Blanco (DECam) and Subaru (HSC).
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Jose Rodriguez Espinosa and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended Data Fig. 1 The HSC-y – DECam-NB964 (and Lyα EW) distribution of LAEs in the LAGER COSMOS field.
The LAEs inside the LAGER-z7OD1 are plotted in blue and those field LAEs in orange. For sources without detection in HSC-y we simply adopt the 2 σ lower limits to their HSC-y magnitudes. Most sources with color > 2 in the plot are non-detected in HSC-y. The vertical lines plot the median colors (1.84 and 1.86) respectively. The tick mark of Lyα EW is derived from the color assuming a redshift of 6.931 (corresponding to the center of NB964 transmission curve).
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Hu, W., Wang, J., Infante, L. et al. A Lyman-α protocluster at redshift 6.9. Nat Astron 5, 485–490 (2021). https://doi.org/10.1038/s41550-020-01291-y
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