2020ApJ...904..144J

Lyα emission from galaxies can be utilized to characterize the ionization state in the intergalactic medium (IGM). We report our search for Lyα emission at z > 7 using a comprehensive Keck/MOSFIRE near-infrared spectroscopic data set, as part of the Texas Spectroscopic Search for Lyα 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 Lyα emission lines detected (>4σ) at z > 7, significantly increasing the spectroscopically confirmed sample. Our sample includes large equivalent-width (EW) Lyα emitters (>50 Å), and additional tentative Lyα emission lines detected at 3σ-4σ from five additional galaxies. We constrain the Lyα EW distribution at z ∼ 7.6, finding a significant drop from z <= 6, suggesting an increasing fraction of neutral hydrogen (H I) in the IGM in this epoch. We estimate the Lyα transmission through the IGM (=EW_z ∼ 7.6_/EWz∼2–6_) and infer an IGM H I fraction (X_HI) of 49_–19⁺¹⁹ % at z ∼ 7.6, which is lower in modest tension (>1σ) with recent measurements at z ∼ 7.6. The spatial distribution of the detected Lyα emitters implies the presence of a potential highly ionized region at z ∼ 7.55, which hosts four Lyα emitters within a ∼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 Lyα EW distribution values and reionization models.