Peering into cosmic reionization: Ly<i>α</i> visibility evolution from galaxies at <i>z</i> = 4.5–8.5 with JWST
Lorenzo Napolitano, L. Pentericci, P. Santini, Antonello Calabrò, Sara Mascia, Mario Llerena, M. Castellano, Mark Dickinson, Steven L. Finkelstein, R. Amorín, Pablo Arrabal Haro, Micaela B. Bagley, Rachana Bhatawdekar, Nikko J. Cleri, Kelcey Davis, Jonathan P. Gardner, Eric Gawiser, Mauro Giavalisco, Nimish P. Hathi, B. W. Holwerda, Weida Hu, Intae Jung, Jeyhan S. Kartaltepe, Anton M. Koekemoer, R. L. Larson, E. Merlin, Bahram Mobasher, Casey Papovich, Hyunbae Park, Nor Pirzkal, Jonathan R. Trump, Stephen M. Wilkins, L. Y. Aaron Yung
Abstract
The resonant scattering interaction between Ly α photons and neutral hydrogen implies that a partially neutral intergalactic medium has the ability to significantly impact the detectability of Ly α emission in galaxies. Thus, the redshift evolution of the Ly α equivalent width distribution of galaxies offers a key observational probe of the degree of ionization during the Epoch of Reionization (EoR). Previous in-depth investigations at z ≥ 7 were limited by ground-based instrument capabilities. We present an extensive study of the evolution of Ly α emission from galaxies at 4.5 < z < 8.5, observed as part of the CEERS and JADES surveys in the JWST NIRSpec/PRISM configuration. The sample consists of 235 galaxies in the redshift range of 4.1 < z < 9.9. We identified 65 of them as Ly α emitters. We first measured the Ly α escape fractions from Ly α to Balmer line flux ratios and explored the correlations with the inferred galaxies’ physical properties, which are similar to those found at lower redshift. We also investigated the possible connection between the escape of Ly α photons and the inferred escape fractions of LyC photons obtained from indirect indicators, finding no secure correlation. We then analyzed the redshift evolution of the Ly α emitter fraction, finding lower average values at z = 5 and 6 compared to previous ground-based observations. At z = 7, the GOODS-S results are aligned with previous findings, whereas the visibility in the EGS field appears to be enhanced. This discrepancy in Ly α visibility between the two fields could potentially be attributed to the presence of early reionized regions in the EGS. Such a broad variance is also expected in the Cosmic Dawn II radiation-hydrodynamical simulation. The average Ly α emitter fraction obtained from the CEERS+JADES data continues to increase from z = 5 to 7, ultimately declining at z = 8. This suggests a scenario in which the ending phase of the EoR is characterized by ∼1 pMpc ionized bubbles around a high fraction of moderately bright galaxies. Finally, we characterize such two ionized regions found in the EGS at z = 7.18 and z = 7.49 by estimating the radius of the ionized bubble that each of the spectroscopically-confirmed members could have created.