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

We present the evolution of Ly alpha 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 alpha emission at the >3 sigma level, and we obtain Ly alpha equivalent width (EW) measurements and stringent 3 sigma upper limits for the 15 and 38 galaxies, respectively. Confirming that Ly alpha velocity offsets and line widths of our galaxies are comparable to those of low-redshift Ly alpha emitters, we investigate the redshift evolution of the Ly alpha EW. We find that Ly alpha EWs statistically decrease toward high redshifts on the Ly alpha 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 alpha 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 similar to 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 alpha 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.