Inferring the IGM Neutral Fraction at z ~ 6-8 with Low-Luminosity Lyman Break Galaxies

Abstract

We present a Bayesian inference on the neutral hydrogen fraction of the intergalactic medium (IGM), $x_{\text{HI}}$, at $z\sim$ 6-8 using the properties of Lyman break galaxies during the Epoch of Reionization. We use large samples of LBG candidates at $5.5\le z\le 8.2$ with spectroscopy from Keck/DEIMOS and Keck/MOSFIRE. For each galaxy, we incorporate either the Lyman-$\hat{I}\pm$ equivalent width (EW) for detections or the EW limit spectrum for nondetections to parameterize the EW distribution at various ultraviolet brightnesses for a given redshift. Using our reference sample of galaxy candidates from the ionized universe at $z$ $\sim$ 6.0, we are able to infer $x_{\text{HI}}$ at two redshifts: $z$ $\sim$ 6.7 and $z$ $\sim$ 7.6. This work includes intrinsically faint, gravitationally lensed galaxies at $z$ $\sim$ 6.0 in order to constrain the intrinsic faint-end Ly$\hat{I}\pm$ EW distribution and provide a comparable population of galaxies to counterparts in our sample that are at higher redshift. The inclusion of faint galaxy candidates, in addition to a more sophisticated modelling framework, allows us to better isolate effects of the interstellar medium and circumgalactic medium on the observed Lyman-$\hat{I}\pm$ distribution from those of the IGM. We infer an upper limit of $x_{\text{HI}}$ $\le$ 0.25 at $z$ = 6.7 $\pm$ 0.2 and a neutral fraction of $x_{\text{HI}}$ = $0.83_{-0.11}^{+0.08}$ at $z$ = 7.6$\pm$ 0.6, both within 1$\ddot{I}ƒ$ uncertainty, results which favor a moderately late and rapid reionization.

11 pages, 7 figures