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A population of ultraviolet-dim protoclusters detected in absorption


Galaxy protoclusters, which will eventually grow into the massive clusters we see in the local Universe, are usually traced by locating overdensities of galaxies1. Large spectroscopic surveys of distant galaxies now exist, but their sensitivity depends mainly on a galaxy’s star-formation activity and dust content rather than its mass. Tracers of massive protoclusters that do not rely on their galaxy constituents are therefore needed. Here we report observations of Lyman-α absorption in the spectra of a dense grid of background galaxies2,3, which we use to locate a substantial number of candidate protoclusters at redshifts 2.2 to 2.8 through their intergalactic gas. We find that the structures producing the most absorption, most of which were previously unknown, contain surprisingly few galaxies compared with the dark-matter content of their analogues in cosmological simulations4,5. Nearly all of the structures are expected to be protoclusters, and we infer that half of their expected galaxy members are missing from our survey because they are unusually dim at rest-frame ultraviolet wavelengths. We attribute this to an unexpectedly strong and early influence of the protocluster environment6,7 on the evolution of these galaxies that reduced their star formation or increased their dust content.

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Fig. 1: Maps of the intergalactic medium.
Fig. 2: The galaxy overdensity δgal at the location of Lyα absorption peaks.
Fig. 3: Average radial profiles of the galaxy or halo overdensity around Lyα absorption peaks.

Data availability

The data supporting the findings of this study are available from the corresponding author upon request.

Code availability

We have made use of public codes including astropy54,55 and dachshund9. Other supporting analysis code is available from the corresponding author upon request.


  1. Overzier, R. A. The realm of the galaxy protoclusters. A review. Astron. Astrophys. Rev. 24, 14 (2016).

    Article  ADS  Google Scholar 

  2. Lee, K.-G., Hennawi, J. F., White, M., Croft, R. A. C. & Ozbek, M. Observational requirements for Lyα forest tomographic mapping of large-scale structure at z 2. Astrophys. J. 788, 49 (2014).

    Article  ADS  CAS  Google Scholar 

  3. Newman, A. B. et al. LATIS: the Lyα Tomography IMACS Survey. Astrophys. J. 891, 147 (2020).

    Article  ADS  CAS  Google Scholar 

  4. Klypin, A., Yepes, G., Gottlöber, S., Prada, F. & Heß, S. MultiDark simulations: the story of dark matter halo concentrations and density profiles. Mon. Not. R. Astron. Soc. 457, 4340–4359 (2016).

    Article  ADS  CAS  Google Scholar 

  5. Nelson, D. et al. The IllustrisTNG simulations: public data release. Comput. Astrophys. Cosmol. 6, 2 (2019).

    Article  ADS  Google Scholar 

  6. Contini, E., De Lucia, G., Hatch, N., Borgani, S. & Kang, X. Semi-analytic model predictions of the galaxy population in protoclusters. Mon. Not. R. Astron. Soc. 456, 1924–1935 (2016).

    Article  ADS  Google Scholar 

  7. Muldrew, S. I., Hatch, N. A. & Cooke, E. A. Galaxy evolution in protoclusters. Mon. Not. R. Astron. Soc. 473, 2335–2347 (2018).

    Article  ADS  CAS  Google Scholar 

  8. Pichon, C., Vergely, J. L., Rollinde, E., Colombi, S. & Petitjean, P. Inversion of the Lyman α forest: three-dimensional investigation of the intergalactic medium. Mon. Not. R. Astron. Soc. 326, 597–620 (2001).

    Article  ADS  Google Scholar 

  9. Stark, C. W., White, M., Lee, K.-G. & Hennawi, J. F. Protocluster discovery in tomographic Ly α forest flux maps. Mon. Not. R. Astron. Soc. 453, 311–327 (2015).

    Article  ADS  CAS  Google Scholar 

  10. Lee, K.-G. et al. First data release of the COSMOS Lyα mapping and tomography observations: 3D Lyα forest tomography at 2.05 < z < 2.55. Astrophys. J. Suppl. Ser. 237, 31 (2018).

    Article  ADS  CAS  Google Scholar 

  11. Lee, K.-G. et al. Shadow of a colossus: a z = 2.44 galaxy protocluster detected in 3D Lyα forest tomographic mapping of the COSMOS field. Astrophys. J. 817, 160 (2016).

    Article  ADS  Google Scholar 

  12. Chiang, Y.-K., Overzier, R. & Gebhardt, K. Discovery of a large number of candidate protoclusters traced by 15 Mpc-scale galaxy overdensities in COSMOS. Astrophys. J. Lett. 782, L3 (2014).

    Article  ADS  Google Scholar 

  13. Chiang, Y.-K. et al. Surveying galaxy proto-clusters in emission: a large-scale structure at z = 2.44 and the outlook for HETDEX. Astrophys. J. 808, 37 (2015).

    Article  ADS  CAS  Google Scholar 

  14. Diener, C. et al. A protocluster at z = 2.45. Astrophys. J. 802, 31 (2015).

    Article  ADS  Google Scholar 

  15. Casey, C. M. et al. A massive, distant proto-cluster at z = 2.47 caught in a phase of rapid formation? Astrophys. J. Lett. 808, L33 (2015).

    Article  ADS  CAS  Google Scholar 

  16. Cucciati, O. et al. The progeny of a cosmic titan: a massive multi-component proto-supercluster in formation at z = 2.45 in VUDS. Astron. Astrophys. 619, A49 (2018).

    Article  CAS  Google Scholar 

  17. Cucciati, O. et al. Discovery of a rich proto-cluster at z = 2.9 and associated diffuse cold gas in the VIMOS Ultra-Deep Survey (VUDS). Astron. Astrophys. 570, A16 (2014).

    Article  Google Scholar 

  18. Lemaux, B. C. et al. VIMOS Ultra-Deep Survey (VUDS): witnessing the assembly of a massive cluster at z 3.3. Astron. Astrophys. 572, A41 (2014).

    Article  Google Scholar 

  19. Blanton, M. R. & Moustakas, J. Physical properties and environments of nearby galaxies. Annu. Rev. Astron. Astrophys. 47, 159–210 (2009).

    Article  ADS  CAS  Google Scholar 

  20. Muzzin, A. et al. The Gemini Cluster Astrophysics Spectroscopic Survey (GCLASS): the role of environment and self-regulation in galaxy evolution at z 1. Astrophys. J. 746, 188 (2012).

    Article  ADS  CAS  Google Scholar 

  21. Cappellari, M. et al. The ATLAS3D project—XX. Mass-size and mass-σ distributions of early-type galaxies: bulge fraction drives kinematics, mass-to-light ratio, molecular gas fraction and stellar initial mass function. Mon. Not. R. Astron. Soc. 432, 1862–1893 (2013).

    Article  ADS  CAS  Google Scholar 

  22. Casey, C. M. The ubiquity of coeval starbursts in massive galaxy cluster progenitors. Astrophys. J. 824, 36 (2016).

    Article  ADS  Google Scholar 

  23. Wang, T. et al. A dominant population of optically invisible massive galaxies in the early Universe. Nature 572, 211–214 (2019).

    Article  ADS  CAS  PubMed  Google Scholar 

  24. Shu, X. et al. A census of optically dark massive galaxies in the Early Universe from magnification by lensing galaxy clusters. Astrophys. J. 926, 155 (2022).

    Article  ADS  Google Scholar 

  25. Wang, T. et al. Discovery of a galaxy cluster with a violently starbursting core at z = 2.506. Astrophys. J. 828, 56 (2016).

    Article  ADS  Google Scholar 

  26. McConachie, I. et al. Spectroscopic confirmation of a protocluster at z = 3.37 with a high fraction of quiescent galaxies. Astrophys. J. 926, 37 (2022).

  27. Zavala, J. A. et al. On the gas content, star formation efficiency, and environmental quenching of massive galaxies in protoclusters at z ≈ 2.0–2.5. Astrophys. J. 887, 183 (2019).

    Article  ADS  CAS  Google Scholar 

  28. Chapman, S. C. et al. Do submillimeter galaxies really trace the most massive dark-matter halos? Discovery of a high-z cluster in a highly active phase of evolution. Astrophys. J. 691, 560–568 (2009).

    Article  ADS  Google Scholar 

  29. Hung, C.-L. et al. Large-scale structure around a z = 2.1 cluster. Astrophys. J. 826, 130 (2016).

    Article  ADS  Google Scholar 

  30. Shi, K. et al. How do galaxies trace a large-scale structure? A case study around a massive protocluster at z = 3.13. Astrophys. J. 879, 9 (2019).

    Article  ADS  CAS  Google Scholar 

  31. Nantais, J. B. et al. Evidence for strong evolution in galaxy environmental quenching efficiency between z = 1.6 and z = 0.9. Mon. Not. R. Astron. Soc. 465, L104–L108 (2017).

    Article  ADS  Google Scholar 

  32. Weaver, J. R. et al. COSMOS2020: a panchromatic view of the Universe to z 10 from two complementary catalogs. Astrophys. J. Suppl. Ser. 258, 11 (2022).

    Article  ADS  Google Scholar 

  33. Cai, Z. et al. Mapping the Most Massive Overdensities through Hydrogen (MAMMOTH). II. Discovery of the extremely massive overdensity BOSS1441 at z = 2.32. Astrophys. J. 839, 131 (2017).

    Article  ADS  CAS  Google Scholar 

  34. Shi, D. D. et al. Spectroscopic confirmation of two extremely massive protoclusters, BOSS1244 and BOSS1542, at z = 2.24. Astrophys. J. 915, 32 (2021).

    Article  ADS  CAS  Google Scholar 

  35. Rakic, O., Schaye, J., Steidel, C. C. & Rudie, G. C. Calibrating galaxy redshifts using absorption by the surrounding intergalactic medium. Mon. Not. R. Astron. Soc. 414, 3265–3271 (2011).

    Article  ADS  CAS  Google Scholar 

  36. Landy, S. D. & Szalay, A. S. Bias and variance of angular correlation functions. Astrophys. J. 412, 64 (1993).

    Article  ADS  Google Scholar 

  37. Roche, N. D., Almaini, O., Dunlop, J., Ivison, R. J. & Willott, C. J. The clustering, number counts and morphology of extremely red (RK > 5) galaxies to K ≤ 21. Mon. Not. R. Astron. Soc. 337, 1282–1298 (2002).

    Article  ADS  Google Scholar 

  38. Trainor, R. F. & Steidel, C. C. The halo masses and galaxy environments of hyperluminous QSOs at z 2.7 in the Keck Baryonic Structure Survey. Astrophys. J. 752, 39 (2012).

    Article  ADS  Google Scholar 

  39. Durkalec, A. et al. The VIMOS Ultra Deep Survey. Luminosity and stellar mass dependence of galaxy clustering at z 3. Astron. Astrophys. 612, A42 (2018).

    Article  CAS  Google Scholar 

  40. Ishikawa, S. et al. The galaxy–halo connection in high-redshift Universe: details and evolution of stellar-to-halo mass ratios of Lyman break galaxies on CFHTLS deep fields. Astrophys. J. 841, 8 (2017).

    Article  ADS  CAS  Google Scholar 

  41. Gunn, J. E. & Peterson, B. A. On the density of neutral hydrogen in intergalactic space. Astrophys. J. 142, 1633–1636 (1965).

    Article  ADS  CAS  Google Scholar 

  42. Weinberg, D. H., Hernsquit, L., Katz, N., Croft, R. & Miralda-Escudé, J. in Structure and Evolution of the Intergalactic Medium from QSO Absorption Line System, Proc. 13th IAP Astrophysics Colloquim (eds Petitjean, P. & Charlot, S.) 133–138 (Editions Frontières, 1997).

  43. Lee, K.-G., Suzuki, N. & Spergel, D. N. Mean-flux-regulated principal component analysis continuum fitting of Sloan Digital Sky Survey Lyα forest spectra. Astron. J. 143, 51 (2012).

    Article  ADS  CAS  Google Scholar 

  44. Bird, S. et al. Reproducing the kinematics of damped Lyman α systems. Mon. Not. R. Astron. Soc. 447, 1834–1846(2015).

    Article  ADS  CAS  Google Scholar 

  45. Bird, S. FSFE: Fake Spectra Flux Extractor. Astrophysics Source Code Library ascl:1710.012 (2017).

  46. Rahmati, A., Pawlik, A. H., Raičević, M. & Schaye, J. On the evolution of the H I column density distribution in cosmological simulations. Mon. Not. R. Astron. Soc. 430, 2427–2445 (2013).

  47. Qezlou, M., Newman, A. B., Rudie, G. C. & Bird, S. Characterizing protoclusters and protogroups at z 2.5 using Lyman-α tomography. Preprint at (2021).

  48. Miller, J. S. A., Bolton, J. S. & Hatch, N. A. Searching for the shadows of giants—II. The effect of local ionization on the Lyα absorption signatures of protoclusters at redshift z 2.4. Mon. Not. R. Astron. Soc. 506, 6001–6013 (2021).

    Article  ADS  CAS  Google Scholar 

  49. Kooistra, R., Inoue, S., Lee, K.-G., Cen, R. & Yoshida, N. Detecting Preheating in Protoclusters with Lyα Forest Tomography. Astrophys. J. 927, 53 (2022).

  50. Cai, Z. et al. Mapping the Most Massive Overdensity Through Hydrogen (MAMMOTH) I: methodology. Astrophys. J. 833, 135 (2016).

    Article  ADS  CAS  Google Scholar 

  51. Zafar, T. et al. The ESO UVES advanced data products quasar sample. II. Cosmological evolution of the neutral gas mass density. Astron. Astrophys. 556, A141(2013).

    Article  Google Scholar 

  52. Lee, K.-G. & White, M. Revealing the z 2.5 cosmic web with 3D Lyα forest tomography: a deformation tensor approach. Astrophys. J. 831, 181 (2016).

    Article  ADS  Google Scholar 

  53. Behroozi, P. S., Wechsler, R. H. & Conroy, C. The average star formation histories of galaxies in dark matter halos from z = 0−8. Astrophys. J. 770, 57 (2013).

    Article  ADS  Google Scholar 

  54. Astropy Collaboration. et al. Astropy: a community Python package for astronomy. Astron. Astrophys. 558, A33 (2013).

    Article  Google Scholar 

  55. Astropy Collaboration. et al. The Astropy Project: building an open-science project and status of the v2.0 core package. Astron. J. 156, 123 (2018).

    Article  ADS  Google Scholar 

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This paper includes data gathered with the 6.5-meter Magellan Telescopes located at Las Campanas Observatory, Chile. We acknowledge the support of the observatory staff. A.B.N., S.B., and B.C.L. acknowledge support from the National Science Foundation under grant numbers 2108014, 2107821 and 1908422, respectively. E.C. acknowledges support from ANID project Basal AFB-170002. This work is based in part on observations obtained with MegaPrime/MegaCam, a joint project of CFHT and CEA/IRFU, at the Canada–France–Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institut National des Science de l’Univers of the Centre National de la Recherche Scientifique (CNRS) of France and the University of Hawaii. Also based in part on data products produced at Terapix available at the Canadian Astronomy Data Centre as part of the Canada–France–Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS.

Author information

Authors and Affiliations



A.B.N., G.C.R. and G.A.B. designed LATIS, obtained telescope access, and, along with V.P. and E.C., conducted the observations. D.D.K. created the data reduction software. A.B.N., M.Q. and S.B. created the mock surveys. A.B.N. processed the observations, created the Lyα and galaxy density maps, and drafted the manuscript. All authors contributed to the interpretation and manuscript preparation.

Corresponding author

Correspondence to Andrew B. Newman.

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Extended data figures and tables

Extended Data Fig. 1 The projected correlation function wp(R) of LATIS galaxies.

We consider galaxies at 2 < z < 3, evaluating wp separately in the COSMOS and CFHTLS-D1 fields, and compare to the the correlation function of dark matter halos with Mvir > Mmin for several values of Mmin indicated in the legend. To reduce the range, the dimensionless wp(R)/R is scaled by (R/h−1cMpc)1.5. Error bars show 1 s.e. Poisson uncertainties.

Extended Data Fig. 2 Comparison of the statistical properties of the LATIS maps and the mock surveys.

a, The distributions of the flux contrast δF, evaluated at each (1 h−1 cMpc)3 map voxel, in the LATIS maps (black curves) and the MDPL2 mock surveys (red bands enclose 68% and 95% of realizations). The dashed curve show the expected distribution from observational noise alone, as estimated from mock surveys of structureless volumes, i.e., with δF = 0 everywhere. b, The cumulative number of absorption peaks in the LATIS maps and the mock surveys. c, The distributions of the galaxy and halo overdensities, evaluated at each map voxel in the observed and mock maps. Altogether the mock surveys accurately match the observed distributions of δF and δgal individually.

Extended Data Fig. 3 Evaluation of the robustness of the δFδhalo connection.

a, Comparison of the trendline distributions, following Fig. 2, derived from mock surveys based on the MDPL2 (dark matter only) and IllustrisTNG300 (including hydrodynamics, galaxy formation, and HCD lines) simulations. For each simulation, curves show percentiles equivalent to 1, 2, 3, and 4 s.d. in a normal distribution. b, The distribution of differences in δF/σmap between absorption peaks in the fiducial TNG mock surveys and the corresponding peaks in mocks that explicitly exclude HCD lines (N > 1017.2 cm−2). c, Comparison of the trendline distributions, following panel a, derived from the fiducial TNG mock surveys and those that exclude HCD lines.

Extended Data Fig. 4 Distributions of the galaxy or halo overdensity around Lyα absorption peaks.

Grey histograms show the distributions of δgal observed around Lyα absorption peaks within an 8 h−1 cMpc aperture following Fig. 2. Green histograms show the distributions of δhalo in the MDPL2 mock surveys, normalized to match the integrals of the corresponding gray histograms. The absorption peaks are split into four bins of δF, highlighting the discrepancy in the strongest absorption peaks.

Extended Data Fig. 5 Evidence of widespread absorption near the strongest absorption peaks.

The distribution of absorption in the 77 sightlines probing the 8 strongest absorption peaks (δF/σmap < −3.8) within 4 h−1 cMpc is shown and compared to the TNG mock surveys. Along each sightline, δF is averaged within \(|\Delta z| < 4\,{h}^{-1}\) cMpc of the absorption peak. The TNG distributions are scaled to match the integral of the LATIS histogram.

Extended Data Fig. 6 The connection of absorption peaks with the matter overdensity and descendant halos.

a, The distribution of the matter density contrast δm, smoothed with a Gaussian kernel having σ = 4 h−1 cMpc and expressed in units of its standard deviation, at the location of absorption peaks in the mock surveys. Bins of δF/σmap are indicated in the legend. The dotted curve shows the global distribution. b, The analogous distributions of the z = 0 descendant halo mass Mvir(z = 0).

Extended Data Fig. 7 The δFδgal trend evaluated using photometric redshifts.

For each LATIS absorption peak in the COSMOS field, a black point indicates the galaxy overdensity estimated using photometric redshifts32. A trendline (black curve) analogous to Fig. 2 is overlaid on the distribution of trendlines derived from mock surveys (green), demonstrating the large uncertainties at low δF. Bands indicate percentiles equivalent to 1, 2, and 3 s.d. in a normal distribution.

Extended Data Table 1 Coordinates and properties of the 8 strongest LATIS absorption peaks having δF/σmap < −3.8

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Newman, A.B., Rudie, G.C., Blanc, G.A. et al. A population of ultraviolet-dim protoclusters detected in absorption. Nature 606, 475–478 (2022).

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