Probing the Cosmic Reionization History with JWST: Gunn–Peterson and Ly <i>α</i> Damping Wing Absorption at 4.5 &lt; <i>z</i> &lt; 13

Abstract We present a statistical analysis of Ly α absorption using 581 galaxies at z = 4.5–13 observed with multiple JWST/NIRSpec spectroscopy programs, including JADES, UNCOVER, CEERS, and GO/DDT. We carefully construct composite spectra binned by redshift with homogeneous UV properties (UV magnitudes, UV slopes, and Ly α equivalent widths) and identify significant Ly α forest signals in galaxies at z ∼ 5–6, which diminish toward higher redshifts. We also find UV continuum breaks at rest-frame 1216 Å that soften beyond z ≳ 6, confirming the effects of cosmic reionization through a self-consistent transition from Gunn–Peterson to Ly α damping wing absorption in galaxies. Fair comparisons of composite spectra with matched UV magnitudes and slopes across redshift reveal that UV-faint galaxies clearly show stronger Ly α absorption than UV-bright galaxies toward high redshift, providing insights into the topological evolution of reionization. We estimate Ly α transmission at the Gunn–Peterson trough and Ly α damping wing absorption by comparing the galaxy spectra to low- z ( z ∼ 2–5) galaxy templates that include galactic and circumgalactic absorption and Ly α emission. Using these measurements together with reionization simulations, we derive volume average neutral hydrogen fractions of 〈 x H I 〉= 0.00 − 0.00 + 0.12 , 0.25 − 0.20 + 0.10 , 0.65 − 0.35 + 0.27 , 1.00 − 0.20 + 0.00 , and 1.00 − 0.40 + 0.00 at z ∼ 5, 6, 7, 9, and 10, respectively. These values broadly align with a reionization history characterized by a rapid transition around z ∼ 7–8, consistent with Ly α emitter observations. While the physical driver of this rapid reionization remains unclear, it may involve the emergence of hidden active galactic nucleus populations and/or the onset of Lyman-continuum escape from galaxies.