JWST Measurements of Neutral Hydrogen Fractions and Ionized Bubble Sizes at z = 7–12 Obtained with Lyα Damping Wing Absorptions in 27 Bright Continuum Galaxies

Abstract

We present volume-averaged neutral hydrogen fractions x _{H I} and ionized bubble radii R _b measured with Lyα damping wing absorption of galaxies at the epoch of reionization. We combine JWST/NIRSpec spectra taken by the CEERS, GO-1433, DDT-2750, and JADES programs and obtain a sample containing 27 bright UV-continuum (M _UV < ‑18.5 mag) galaxies at 7 < z < 12. We construct four composite spectra binned by redshift and find the clear evolution of the softening break toward high redshift at rest-frame 1216 Å, suggesting an increase of Lyα damping wing absorption. We estimate Lyα damping wing absorption in the galaxy spectra with realistic templates including Lyα emission and circumgalactic medium absorptions. Assuming the standard inside-out reionization picture having an ionized bubble with radius R _b around a galaxy embedded in the intergalactic medium with x _{H I}, we obtain x _{H I} (R _b ) values generally increasing (decreasing) from xHI=0.53‑0.47+0.18 to 0.92‑0.10+0.08 ( logRb=1.67‑0.16+0.14 to ‑0.69‑0.24+0.89 comoving Mpc) at redshift 7.12‑0.08+0.06 to 9.91‑1.15+1.49 . The redshift evolution of x _{H I} indicates a moderately late reionization history consistent with the one previously suggested from the electron scattering of cosmic microwave background and the evolution of UV luminosity function with an escape fraction f _esc ∼ 0.2. Our R _b measurements suggest that bubble sizes could be up to a few dex larger than the cosmic average values estimated by analytic calculations for a given x _{H I}, while our R _b measurements are roughly comparable with the values for merged ionized bubbles around bright galaxies predicted by recent numerical simulations.