Lyα visibility from z = 4.5 to 11 in the UDS field: Evidence for a high neutral hydrogen fraction and small ionized bubbles at z ∼ 7

¹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monteporzio Catone, Italy
² Dipartimento di Fisica, Università di Roma Sapienza, Città Universitaria di Roma – Sapienza, Piazzale Aldo Moro, 2, 00185 Roma, Italy
³ NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Ave., Tucson, AZ 85719, USA
⁴ Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
⁵ Department of Astronomy, The University of Texas at Austin, Austin, TX, USA
⁶ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁷ University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, MA 01003-9305, USA
⁸ Institute of Physics, Laboratory of Galaxy Evolution, Ecole Polytechnique Federale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁰ Institute of Science and Technology Austria (ISTA), Am Campus 1, A-3400 Klosterneuburg, Austria
¹¹ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4 Torrejón de Ardoz, E-28850, Madrid, Spain
¹² University of Ljubljana FMF, Jadranska 19, 1000 Ljubljana, Slovenia
¹³ IFPU – Institute for Fundamental Physics of the Universe, via Beirut 2, I-34151 Trieste, Italy
¹⁴ MMT/Steward Observatory, University of Arizona, 933 N. Cherry St., Tucson, AZ 85721, USA
¹⁵ Max Planck Institut für Astrophysik, Karl Schwarzschild Straße 1, D-85741 Garching, Germany
¹⁶ School of Astronomy and Space Science, University of Chinese Academy of Sciences (UCAS), Beijing 100049, China
¹⁷ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
¹⁸ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China

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Received: 19 August 2025
Accepted: 21 February 2026

Abstract

The resonant scattering nature of Lyα photons interacting with neutral hydrogen makes Lyα emitters (LAEs) robust tracers of the intergalactic neutral hydrogen fraction, and thus sensitive probes of cosmic reionization. We present an extensive study of the Lyα evolution from galaxies at 4.5 ≤ z ≤ 11 in the UDS field, observed as part of the CAPERS survey, and complemented with spectra from the DAWN JWST Archive. The combined sample includes 651 spectroscopically confirmed Lyα-break galaxies, among which we find 73 S/N > 3 LAEs in JWST-NIRSpec PRISM spectra. We trace the redshift evolution of the LAE fraction with EW₀ > 25 Å (X_Lyα) between z = 5 and z = 9, extending such an analysis to the UDS field for the first time. At z = 5 and 6, the UDS results agree with the average JWST X_Lyα values from multiple fields. However, JWST measurements are consistently lower than ground-based results. To investigate this, we compare JWST observations to a population of star-forming galaxies at z ∼ 6 observed with VLT-FORS2. We find that a Lyα slit-loss of 35 ± 10% in JWST spectra accounts for the offset, as the resonant Lyα emission is more spatially extended than the stellar continuum. From z = 6 to 7, the UDS field shows a significant drop in Lyα visibility, from which we infer a neutral hydrogen fraction of X_HI = 0.7–0.9. Finally, we identify two robust ionized bubbles at z = 7.29 and 7.77, with radii of R_ion = 0.6 and 0.5 physical Mpc and photometric overdensities of N/⟨N⟩ = 3 and 4, based on candidate counts down to the photometric completeness limit. Compared to the large ionized region at z ∼ 7 in the EGS field, these results indicate significant field-to-field variation, supporting a patchy, inhomogeneous reionization process.