JADES: The emergence and evolution of Ly emission and constraints on the intergalactic medium neutral fraction

The rest-frame UV recombination emission line Ly alpha can be powered by ionising photons from young massive stars in star-forming galaxies, but the fact that it can be resonantly scattered by neutral gas complicates its interpretation. For reionisation-era galaxies, a neutral intergalactic medium will scatter Ly alpha from the line of sight, making Ly alpha a useful probe of the neutral fraction evolution. Here, we explore Ly alpha in JWST/NIRSpec spectra from the ongoing JADES programme, which targets hundreds of galaxies in the well-studied GOODS-S and GOODS-N fields. These sources are UV-faint (-20.4 < M-UV < -16.4) and thus represent a poorly explored class of galaxy. We fitted the low spectral resolution spectra (R similar to 100) of a subset of 84 galaxies in GOODS-S with z(spec) > 5.6 (as derived with optical lines) with line and continuum models to search for significant line emission. Through exploration of the R100 data, we find evidence for Ly alpha in 17 sources. This sample allowed us to place observational constraints on the fraction of galaxies with Ly alpha emission in the redshift range 5.6 < z < 7.5, with a decrease from z = 6 to z = 7. We also find a positive correlation between the Ly alpha equivalent width and MUV, as seen in other samples. We used these results to estimate the neutral gas fraction at z similar to 7, and our estimates are in agreement with previous results (X-HI similar to 0.5 - 0.9).