Constraining the Neutral Fraction of Hydrogen in the IGM at Redshift 7.5

Abstract

We present a large spectroscopic campaign with Keck/Multi-Object Spectrometer for InfraRed Exploration (MOSFIRE) targeting Lyα emission (Lyα) 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 Lyα equivalent-width limits. During the campaign, we targeted 70 LBG candidates with the MOSFIRE Y band, selected photometrically to cover Lyα over the range 7 < z < 8.2. We detect signal-to-noise ratio > 5 emission lines in two of these galaxies and find that they are likely Lyα at z = 7.148 ± 0.001 and z = 7.161 ± 0.001. We present new lens models for four 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}}_{{H}{{I}}}, in the IGM during reionization to be {\overline{x}}_{{H}{{I}}}={0.88}_-0.10^+0.05 at z = 7.6 ± 0.6. Our result is consistent with a late and rapid reionization scenario inferred by Planck.