Lyα Emission at z = 7–13: Clear Evolution of Lyα Equivalent Width Indicating a Late Cosmic Reionization History

May 2024 • 2024ApJ...967...28N

Authors • Nakane, Minami • Ouchi, Masami • Nakajima, Kimihiko • Harikane, Yuichi • Ono, Yoshiaki • Umeda, Hiroya • Isobe, Yuki • Zhang, Yechi • Xu, Yi

Abstract • We present the evolution of Lyα emission derived from 53 galaxies at z = 6.6–13.2, which have been identified by multiple JWST/NIRSpec spectroscopy programs of Early Release Science, General Observer, Director's Discretionary Time, and Guaranteed Time Observations. These galaxies fall on the star formation main sequence and are typical star-forming galaxies with UV magnitudes of ‑22.5 ≤ M _UV ≤ ‑17.0. We find that 15 out of 53 galaxies show Lyα emission at the >3σ level, and we obtain Lyα equivalent width (EW) measurements and stringent 3σ upper limits for the 15 and 38 galaxies, respectively. Confirming that Lyα velocity offsets and line widths of our galaxies are comparable to those of low-redshift Lyα emitters, we investigate the redshift evolution of the Lyα EW. We find that Lyα EWs statistically decrease toward high redshifts on the Lyα EW versus the M _UV plane for various probability distributions of the uncertainties. We then evaluate neutral hydrogen fractions x _{H I} with the redshift evolution of the Lyα EW and the cosmic reionization simulation results on the basis of a Bayesian inference framework, and obtain x _{H I} < 0.79, $={0.62}_{-0.36}^{+0.15}$ <?MML = 0.62 ‑ 0.36 + 0.15 ?> , and ${0.93}_{-0.07}^{+0.04}$ <?MML 0.93 ‑ 0.07 + 0.04 ?> at z ∼ 7, 8, and 9–13, respectively. These moderately large x _{H I} values are consistent with the Planck cosmic microwave background optical depth measurement and previous x _{H I} constraints from galaxy and QSO Lyα damping wing absorption and strongly indicate a late reionization history. Such a late reionization history suggests that major sources of reionization would emerge late and be hosted by moderately massive halos compared with the widely accepted picture of abundant low-mass objects for the sources of reionization.

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Yechi Zhang

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