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

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 (XLyα) 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 XLyα 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 XHI = 0.7–0.9. Finally, we identify two robust ionized bubbles at z = 7.29 and 7.77, with radii of Rion = 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.