All the massive galaxy overdensities during reionisation

JWST rest-frame optical selection reveals young, chemically evolved galaxies embedded in dense, neutral gas at z > 5

¹ Cosmic Dawn Center (DAWN), Denmark
² Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
³ DTU Space, Technical University of Denmark, Elektrovej 327, DK2800 Kgs. Lyngby, Denmark
⁴ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria
⁵ MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139, USA
⁶ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁷ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 13 February 2026
Accepted: 6 May 2026

Abstract

The high-redshift progenitors of present-day galaxy clusters are believed to substantially contribute to the global star formation rate density and drive the large-scale reionisation of the Universe. Here we present a blind and unbiased search for and characterisation of galaxy overdensities during the reionisation epoch at redshifts z ∼ 5.5 − 7 based on rest-frame optical JWST/NIRCam grism spectroscopy of the Abell 2744 lensing field as part of the JWST All the Little Things (ALT) survey. Using a physically motivated, cosmological inference friends-of-friends (FoF) algorithm, we identified six galaxy overdensities, including five robust systems at z = 5.66–6.77. They are all characterised by total halo masses of M_halo ≳ 10¹¹ M_⊙, inferred from a range of proxies. We find that the galaxy members in these overdense environments are on average less massive though equally metal-rich, and generally comprised of younger stellar populations, as indicated by their bluer spectral slopes and less prominent Balmer breaks compared to field galaxies at similar redshifts. Further, we use this novel rest-frame optical selection of galaxy proto-clusters to infer the fraction and 3D distribution of strong Lyman-α emitters (LAEs) and damped Lyman-α absorbers (DLAs) in the overdensity environments. We find that two out of the six galaxy overdensities have excess H I absorption compared to the field average, while the other four are consistent within their large scatter in density. These results present the first direct observational constraints on the tomography of the dense, neutral gas reservoirs in large-scale galaxy overdensities at z > 5 and highlight the limitations of pre-JWST searches for reionisation-era galaxy overdensities relying on the detection of strong LAEs alone.