# Constraining the neutral fraction of hydrogen in the IGM at redshift 7.5

## TL;DR

This study uses spectroscopic observations of faint galaxies behind galaxy clusters to measure the neutral hydrogen fraction in the early universe, providing insights into the timeline of cosmic reionization.

## Contribution

It presents new spectroscopic data and lens models, and employs Bayesian analysis with simulations to constrain the IGM's neutral hydrogen fraction at redshift 7.6.

## Key findings

- Neutral hydrogen fraction at z=7.6 is approximately 0.88.
- Detected Lyα emission in two faint galaxies at z~7.15.
- Results support a late and rapid reionization scenario.

## Abstract

We present a large spectroscopic campaign with Keck/MOSFIRE targeting Lyman-alpha emission (Ly$\alpha$) from intrinsically faint Lyman-break Galaxies (LBGs) behind 12 efficient galaxy cluster lenses. Gravitational lensing allows us to probe the more abundant faint galaxy population to sensitive Ly$\alpha$ equivalent width limits. During the campaign we targeted 70 LBG candidates with MOSFIRE Y-band, selected photometrically to cover Ly$\alpha$ over the range $7<z<8.2$. We detect $S/N>5$ emission lines in 2 of these galaxies and find that they are likely Ly$\alpha$ at $z=7.148\pm0.001$ and $z=7.161\pm0.001$. We present new lens models for 4 of the galaxy clusters, using our previously published lens models for the remaining clusters to determine the magnification factors for the source galaxies. Using a Bayesian framework that employs large scale reionization simulations of the intergalactic medium (IGM) as well as realistic properties of the interstellar medium and circumgalactic medium, we infer the volume-averaged neutral hydrogen fraction, $\overline{x}_{\mathrm{HI}}$, in the IGM during reionization to be $\overline{x}_{\mathrm{HI}}=0.88^{+0.05}_{-0.10}$ at $z=7.6\pm0.6$. Our result is consistent with a late and rapid reionization scenario inferred by Planck.