Peering into cosmic reionization: Ly alpha visibility evolution from galaxies at z = 4.5 - 8.5 with JWST

Abstract

The resonant scattering interaction between photons and neutral hydrogen implies that a partially neutral intergalactic medium has the ability to significantly impact the detectability of emission in galaxies. Thus, the redshift evolution of the 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 $z$ geq 7 were limited by ground-based instrument capabilities. We present an extensive study of the evolution of 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 emitters. We first measured the escape fractions from 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 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 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 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 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 sim 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.