Ly α 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

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Abstract
The resonant scattering nature of Ly α photons interacting with neutral hydrogen makes Ly α emitters (LAEs) robust tracers of the intergalactic neutral hydrogen fraction, and thus sensitive probes of cosmic reionization. We present an extensive study of the Ly α 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 Ly α -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 EW 0 > 25 Å (X Ly α ) 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 Ly α 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 Ly α slit-loss of 35 ± 10% in JWST spectra accounts for the offset, as the resonant Ly α emission is more spatially extended than the stellar continuum. From z = 6 to 7, the UDS field shows a significant drop in Ly α 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.

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