Ly$\alpha$ Emission at $z=7-13$: Clear Ly$\alpha$ Equivalent Width Evolution Indicating the Late Cosmic Reionization History

TL;DR

This study analyzes Ly$ extalpha$ emission from galaxies at redshifts 6.6 to 13.2 using JWST data, revealing a decline in Ly$ extalpha$ equivalent widths with increasing redshift, which indicates a late cosmic reionization history.

Contribution

It provides new measurements of Ly$ extalpha$ emission at high redshifts and infers the neutral hydrogen fraction, supporting a late reionization scenario with implications for reionization sources.

Findings

Ly$ extalpha$ EWs decrease with redshift, indicating evolving IGM conditions.

Neutral hydrogen fraction $x_{HI}$ increases at higher redshifts, suggesting late reionization.

Results are consistent with Planck CMB and previous damping wing constraints.

Abstract

We present the evolution of Ly$\alpha$ emission derived from 53 galaxies at $z=6.6-13.2$ that are identified by multiple JWST/NIRSpec spectroscopy programs of ERS, GO, DDT, and GTO. These galaxies fall on the star-formation main sequence and are the typical star-forming galaxies with UV magnitudes of $-22.5\leq M_\mathrm{UV}\leq-17.0$. We find that 15 out of 53 galaxies show Ly$\alpha$ emission at the $>3\sigma$ levels, and 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 with 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 towards high redshifts on the Ly$\alpha$ EW vs. $M_{\rm UV}$ plane for various probability distributions of the uncertainties. We then evaluate neutral hydrogen fractions $x_{\rm HI}$ with the Ly$\alpha$ EW redshift evolution and the cosmic reionization simulation results on the basis of a Bayesian inference framework, and obtain $x_{\rm HI}<0.79$, $=0.62^{+0.15}_{-0.36}$, and $0.93^{+0.04}_{-0.07}$ at $z\sim7$, $8$, and $9-13$, respectively. These moderately large $x_{\rm HI}$ values are consistent with the Planck CMB optical depth measurement and previous $x_{\rm HI}$ constraints from galaxy and QSO Ly$\alpha$ damping wing absorptions, 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 in contrast with the widely-accepted picture of abundant low-mass objects for the sources of reionization.