Ly$\hat{I}\pm$ visibility from z = 4.5 to 11 in the UDS field: Evidence for a high neutral hydrogen fraction and small ionized bubbles at z $\sim$ 7

Abstract

The resonant scattering nature of Ly$\hat{I}\pm$ photons interacting with neutral hydrogen makes Ly$\hat{I}\pm$ 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$\hat{I}\pm$ evolution from galaxies at 4.5 $\le$ z $\le$ 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$\hat{I}\pm$-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 A (X${}_{\mathrm{Ly}\hat{\mathrm{I}}\pm }$) 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${}_{\mathrm{Ly}\hat{\mathrm{I}}\pm }$ 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$\sim$6 observed with VLT-FORS2. We find that a Ly$\hat{I}\pm$ slit-loss of 35 $\pm$ 10% in JWST spectra accounts for the offset, as the resonant Ly$\hat{I}\pm$ emission is more spatially extended than the stellar continuum. From z = 6 to 7, the UDS field shows a significant drop in Ly$\hat{I}\pm$ visibility, from which we infer a neutral hydrogen fraction of X${}_{\mathrm{HI}}$ = 0.7--0.9. Finally, we identify two robust ionized bubbles at z = 7.29 and 7.77, with radii of $R_{\mathrm{ion}}$ = 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$\sim$7 in the EGS field, these results indicate significant field-to-field variation, supporting a patchy, inhomogeneous reionization process.

18 pages, 7 figures, 2 tables. Accepted for publication in A&A