Characterising Lyα damping wings at the onset of reionisation

Evidence for highly efficient star formation driven by dense, neutral gas in UV-bright galaxies at z > 9

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 for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁵ Canadian Institute for Theoretical Astrophysics, 60 St George St, University of Toronto, Toronto, ON M5S 3H8, Canada
⁶ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St George St, Toronto, ON M5S 3H4, Canada
⁷ Department of Physics, 60 St George St, University of Toronto, Toronto, ON M5S 3H8, Canada
⁸ Department of Astronomy, Yale University, New Haven, CT 06511, USA
⁹ Stockholm University, Department of Astronomy and Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, SE-10691 Stockholm, Sweden
¹⁰ Department of Physics, School of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
¹¹ Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
¹² International Centre for Radio Astronomy Research (ICRAR), M468, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
¹³ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁴ MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139, USA
¹⁵ Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA, Leiden, The Netherlands
¹⁶ School of Physics and Astronomy, University of Leicester, University Rd, Leicester LE1 7RH, UK
¹⁷ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁸ Cavendish Laboratory, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK
¹⁹ Max-Planck-Institut fÜr Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

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

Received: 12 February 2026
Accepted: 19 May 2026

Abstract

One of the major conundrums in contemporary extragalactic astrophysics is the apparent overabundance of a remarkable population of UV-bright galaxies at redshifts z ≳ 9. We analysed galaxies spectroscopically observed by JWST/NIRSpec PRISM and confirmed to lie at z > 9, with a sufficient signal-to-noise to carefully model their rest-frame UV-to-optical continua and line emission. In particular, we modelled the damped Lyman-α (Lyα) absorption (DLA) features of each galaxy to place observational constraints on the gas assembly of neutral atomic hydrogen (H I) onto the galaxy halos at the onset of cosmic reionisation. Based on the derived H I column densities and star formation rate (SFR) surface densities, we show that all galaxies are highly efficient at forming stars on rapid ∼10 − 100 Myr depletion timescales, greatly in excess compared to the canonical local Universe Kennicutt-Schmidt (KS) relation and predictions from state-of-the-art galaxy formation simulations. The dense H I gas appears to also drive the offset from the fundamental-metallicity relation (FMR) of these galaxies though its dust-to-gas (DTG) ratio is seemingly consistent with values derived for local galaxies except for the lowest metallicity sight lines. Our results provide the first robust observational constraints on the impact of pristine H I gas on early galaxy assembly, and they imply that a combination of highly efficient star formation and low dust obscuration can likely explain the UV brightness of galaxies at cosmic dawn.