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), xHI, at z 6-8 using the properties of Lyman break galaxies during the Epoch of Reionization. We use large samples of LBG candidates at 5.5 ≤ z ≤ 8.2 with spectroscopy from Keck/DEIMOS and Keck/MOSFIRE. For each galaxy, we incorporate either the Lyman-α 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 6.0, we are able to infer xHI at two redshifts: z 6.7 and z 7.6. This work includes intrinsically faint, gravitationally lensed galaxies at z 6.0 in order to constrain the intrinsic faint-end Lyα 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-α distribution from those of the IGM. We infer an upper limit of xHI ≤ 0.25 at z = 6.7 0.2 and a neutral fraction of xHI = 0.83+0.08-0.11 at z = 7.6 0.6, both within 1σ uncertainty, results which favor a moderately late and rapid reionization.
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