Texas Spectroscopic Search for Lyα Emission at the End of Reionization. III. The Lyα Equivalent-width Distribution and Ionized Structures at z > 7

Ly alpha emission from galaxies can be utilized to characterize the ionization state in the intergalactic medium (IGM). We report our search for Ly alpha emission at z > 7 using a comprehensive Keck/MOSFIRE near-infrared spectroscopic data set, as part of the Texas Spectroscopic Search for Ly alpha Emission at the End of Reionization Survey. We analyze data from 10 nights of MOSFIRE observations which together target 72 high-z candidate galaxies in the GOODS-N field, all with deep exposure times of 4.5-19 hr. Utilizing an improved automated emission-line search, we report 10 Ly alpha emission lines detected (>4 sigma) at z > 7, significantly increasing the spectroscopically confirmed sample. Our sample includes large equivalent-width (EW) Ly alpha emitters (>50 A), and additional tentative Ly alpha emission lines detected at 3 sigma-4 sigma from five additional galaxies. We constrain the Ly alpha EW distribution at z similar to 7.6, finding a significant drop from z less than or similar to 6, suggesting an increasing fraction of neutral hydrogen (H i) in the IGM in this epoch. We estimate the Ly alpha transmission through the IGM (=EWz similar to 7.6/EWz similar to 2-6) and infer an IGM H i fraction (X-HI) of 49(-19)(+19) % z similar to 7.6, which is lower in modest tension (>1 sigma) with recent measurements at z similar to 7.6. The spatial distribution of the detected Ly alpha emitters implies the presence of a potential highly ionized region at z similar to 7.55, which hosts four Ly alpha emitters within a similar to 40 cMpc spatial separation. The prominence of this ionized region in our data set could explain our lower inferred value of X-HI, though our analysis is also sensitive to the chosen reference Ly alpha EW distribution values and reionization models.