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

Abstract

We present the evolution of Ly α 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 Ly α emission at the >3 σ level, and we obtain Ly α equivalent width (EW) measurements and stringent 3 σ upper limits for the 15 and 38 galaxies, respectively. Confirming that Ly α velocity offsets and line widths of our galaxies are comparable to those of low-redshift Ly α emitters, we investigate the redshift evolution of the Ly α EW. We find that Ly α EWs statistically decrease toward high redshifts on the Ly α 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 Ly α EW and the cosmic reionization simulation results on the basis of a Bayesian inference framework, and obtain x _H I < 0.79, $={0.62}_{-0.36}^{+0.15}$ , and ${0.93}_{-0.07}^{+0.04}$ 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 Ly α 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.

Keywords

• Galaxy evolution
• High-redshift galaxies
• Lyα galaxies
• Reionization