Inferring the intergalactic medium neutral fraction at z similar to 6-8 with low-luminosity Lyman break galaxies

We present a Bayesian inference on the neutral hydrogen fraction of the intergalactic medium (IGM), (x) over bar (HI), at z similar to 6-8 using the properties of Lyman break galaxies (LBGs) during the epoch of reionization. We use large samples of I.BG candidates at 5.5 <= z <= 8.2 with spectroscopy from Keck/DEIMOS and Keck/MOSFIRE. For each galaxy, we incorporate either the Lyman-alpha (Ly alpha) equivalent width (EW) for detections or the EW limit spectrum for non-detections to parametrize the EW distribution at various ultraviolet brightnesses for a given redshift. Using our reference sample of galaxy candidates from the ionized universe at z similar to 6.0, we are able to infer (x) over bar (HI) at two redshifts: z similar to 6.7 and z similar to 7.6. This work includes intrinsically faint, gravitationally lensed galaxies at z similar to 6.0 in order to constrain the intrinsic faint-end Ly alpha 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 Ly alpha distribution from those of the IGM. We infer an upper limit of (x) over bar (HI) <= 0.25 (0.44) at z = 6.7 +/- 0.2 and a neutral fraction of (x) over bar (HI) = 0.83(-0.11)(+0.08) (0.83(-0.21)(+0.11)) at z = 7.6 +/- 0.6, both within 68 per cent (95 per cent) uncertainty, results that favour a moderately late and fairly rapid reionization.