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A dense web of neutral gas in a galaxy proto-cluster post-reionization

Nature Astronomy (2026)Cite this article

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Abstract

Galaxy clusters are the most massive, gravitationally bound structures in the Universe. They emerged through hierarchical structure formation of large-scale dark matter and baryon overdensities. Early galaxy ‘proto-clusters’ are believed to have substantially contributed to the cosmic star-formation rate density and served as ‘hotspots’ for the reionization of the intergalactic medium. Our understanding of the formation of these structures at the earliest cosmic epochs is, however, limited to sparse observations of their galaxy members or is based on phenomenological models and cosmological simulations. Here we report the detection of a large and coherent structure of neutral atomic hydrogen gas (H i) extending from a galaxy proto-cluster at redshift z = 5.4, one billion years after the Big Bang. The presence of this H i gas is revealed by strong damped Lyman-α absorption features observed in several background-galaxy spectra. Although the sight lines overall probe a large range in H i column densities, NHI = 1020 cm−2 to 1023.5 cm−2, they are similar across nearby sight lines, demonstrating that they probe the same dense neutral gas. This observation of a dense large-scale overdensity of cold neutral gas challenges current cosmological simulations and has strong implications for the reionization topology of the Universe.

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Fig. 1: JWST/NIRSpec prism spectra of the 12 galaxies with apparent strong DLAs, in close on-sky proximity to the massive galaxy proto-cluster at redshift z = 5.387.
Fig. 2: Spatial and redshift distribution of the galaxy proto-cluster members and the background galaxies with strong DLAs.
Fig. 3: Simulated density maps of H i column density NHI for a proto-cluster with halo mass Mh,200 = 5.5 × 1012 M⊙ at z = 5.53.

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Data availability

The JWST imaging and spectroscopic data are publicly available via the JWST MAST archive at https://mast.stsci.edu. The relevant programme and source IDs for each target are provided in Table 1. The reduced spectroscopic data are all available via DJA at https://dawn-cph.github.io/dja/. Version 3 was used for this work.

Code availability

The data have been processed using the following public software codes: grizli v.1.9.1160 and MsaExp v.0.6.1732.

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Acknowledgements

This work has received funding from the Swiss State Secretariat for Education, Research and Innovation (Contract No. MB22.00072). The Cosmic Dawn Center (DAWN) is funded by the Danish National Research Foundation (Grant No. DNRF140). The data products presented herein were retrieved from the DJA, which is an initiative of the Cosmic Dawn Center. This work is based on observations made with the NASA/ESA/CSA JWST. The data were obtained from MAST at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. J.S.B. acknowledges support from the Simons Collaboration on Learning the Universe. J.S.B.’s simulations used resources from the Cambridge Service for Data Driven Discovery operated by the University of Cambridge Research Computing Service (www.csd3.cam.ac.uk), provided by Dell EMC and Intel using tier 2 funding from the Engineering and Physical Sciences Research Council (Capital Grant No. EP/P020259/1). K.F. gratefully acknowledges support from the National Science Foundation (Award No. 2006550). M.J.H. is fellow of the Knut & Alice Wallenberg Foundation. D.S. acknowledges support from the Science and Technology Facilities Council. U.S.K. was partially funded by the Summer Undergraduate Research Fellowships programme at Caltech.

Author information

Authors and Affiliations

  1. Cosmic Dawn Center (DAWN), Copenhagen, Denmark

    Kasper E. Heintz, Pascal A. Oesch, Albert Sneppen, Clara L. Pollock, Simone Vejlgaard, Chamilla Terp, Gabriel B. Brammer, Kristian Finlator, Francesco Valentino, Peter Laursen & Darach J. Watson

  2. Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

    Kasper E. Heintz, Pascal A. Oesch, Albert Sneppen, Clara L. Pollock, Simone Vejlgaard, Chamilla Terp, Gabriel B. Brammer, Kristian Finlator, Peter Laursen & Darach J. Watson

  3. Department of Astronomy, University of Geneva, Versoix, Switzerland

    Kasper E. Heintz, Pascal A. Oesch & Alba Covelo-Paz

  4. Center for Astrophysics, Harvard University, Cambridge, MA, USA

    Jake S. Bennett

  5. Institute of Astronomy, University of Cambridge, Cambridge, UK

    Jake S. Bennett & Debora Sijacki

  6. Kavli Institute for Cosmology Cambridge, University of Cambridge, Cambridge, UK

    Jake S. Bennett, Joris Witstok & Debora Sijacki

  7. Center for Computational Astrophysics, Flatiron Institute, New York, NY, USA

    Douglas Rennehan

  8. Cavendish Laboratory, University of Cambridge, Cambridge, UK

    Joris Witstok

  9. Astrophysics Research Institute, Liverpool John Moores University, Liverpool, UK

    Renske Smit

  10. California Institute of Technology, Pasadena, CA, USA

    Umran S. Koca

  11. New Mexico State University, Las Cruces, NM, USA

    Kristian Finlator

  12. Department of Astronomy and Oskar Klein Centre for Cosmoparticle Physics, Stockholm University, Stockholm, Sweden

    Matthew J. Hayes

  13. MIT Kavli Institute for Astrophysics and Space Research, Cambridge, MA, USA

    Rohan P. Naidu

  14. Institute of Science and Technology Austria (ISTA), Klosterneuburg, Austria

    Jorryt Matthee

  15. European Southern Observatory, Garching, Germany

    Francesco Valentino

  16. Department of Physics & Astronomy and Leicester Institute of Space & Earth Observation, University of Leicester, Leicester, UK

    Nial R. Tanvir

  17. Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Reykjavik, Iceland

    Páll Jakobsson

  18. Institute for Astronomy, Royal Observatory, University of Edinburgh, Edinburgh, UK

    Romeel Davé & Laura C. Keating

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Contributions

K.E.H. wrote the paper, led the analysis, and produced Figs. 1 and 2 and Supplementary Figs. 1, 2, 4 and 5. G.B.B. reduced and extracted the photometric and spectroscopic data. R.S. and J.W. identified the first evidence for strong DLA in the target galaxies. P.A.O. led the FRESCO observations and analysis of the galaxy proto-cluster members. C.L.P., S.V., C.T. and U.S.K. performed the Lyα modelling of the spectra. C.L.P. produced Supplementary Fig. 3. J.S.B. and D.R. performed the simulations and extracted the relevant data. J.S.B. produced Fig. 3, and D.R. produced Supplementary Fig. 6. A.S. did the statistical clustering analysis and produced Extended Data Fig. 1. All authors contributed to the text and interpretations of the results.

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Correspondence to Kasper E. Heintz.

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Extended data

Extended Data Fig. 1 Relative line-of-sight HI column densities as a function of projected distance at z=5.4 for all background-galaxy pairs.

The right hand panel indicates the CDF of N_HI-ratios. The dashed line indicates the median of the observed distribution of N_HI ratio for galaxies with projected distances between 100 kpc and 1000 kpc. Error bars denote the quadratic uncertainty on the column density ratio. The galaxies associated with the smallest on-sky projected distance display similar column-densities. Objects with projected distance d < 100, d > 100 kpc are indicated in black and blue respectively.

Extended Data Table 1 Bayesian model statistics for the DLA sample
Full size table

Supplementary information

Supplementary Information

Supplementary Figs 1–6.

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Heintz, K.E., Bennett, J.S., Oesch, P.A. et al. A dense web of neutral gas in a galaxy proto-cluster post-reionization. Nat Astron (2026). https://doi.org/10.1038/s41550-025-02745-x

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