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# A population of ultraviolet-dim protoclusters detected in absorption

## Abstract

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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## 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.

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## Acknowledgements

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

### Contributions

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.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

## 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 δF−δhalo 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 δF–δgal 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.

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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). https://doi.org/10.1038/s41586-022-04681-6

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• DOI: https://doi.org/10.1038/s41586-022-04681-6