Probing the Cosmic Reionization History with JWST: Gunn–Peterson and Ly <i>α</i> Damping Wing Absorption at 4.5 < <i>z</i> < 13
Hiroya Umeda, Masami Ouchi, Yuta Kageura, Yuichi Harikane, Minami Nakane, Tran Thi Thai, Kimihiko Nakajima
Abstract
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 〉= <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.00</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.00</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.12</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.25</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.20</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>0.65</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.35</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.27</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>1.00</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.20</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.00</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mn>1.00</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.40</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.00</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> 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.