Toward a precise determination of the neutral gas fraction at 7 using the Lyman alpha fraction test

In recent years, early star-forming galaxies have emerged as one of the most promising probes of the reionization era. A particularly valuable method is Lyman alpha fraction test, which tracks the fraction of color-selected Lyman break galaxies that show strong Lyman alpha in emission across cosmic time. Our previous work, along with several independent studies, identified a marked decline in this fraction beyond z ~ 6.5, thus providing important evidence that we may be directly observing the final stages of reionization. Yet until now, the method for inferring a neutral gas fraction from this Lyman alpha data has been rather imprecise. Earlier studies failed to take into account evolution in specific galaxy properties that may influence the visibility of Lyman alpha apart from any evolution in the intergalactic medium. These include variations in the intrinsic production of Lyman alpha and the emission line offset from the galaxy's systemic velocity. Here we present a comprehensive analysis of these factors affecting the derived neutral hydrogen fraction, and construct a predictive model for the intrinsic Lyman alpha emission taking into account galaxy luminosity, size, and most importantly, ultraviolet color. Using our improved model, we reassess the implications of the most recent data for the ionized state of the intergalactic medium at z ~ 7-8.