Texas Spectroscopic Search for Ly$\alpha$ Emission at the End of Reionization III. the Ly$\alpha$ 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 dataset, 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 ($>$50\r{A}), and additional tentative Ly$\alpha$ emission lines detected at 3 - 4$\sigma$ from five additional galaxies. We constrain the Ly$\alpha$ EW distribution at $z\sim7.6$, finding a significant drop from $z\lesssim6$, suggesting an increasing fraction of neutral hydrogen (HI) in the IGM in this epoch. We estimate the Ly$\alpha$ transmission through the IGM ($=$EW$_{z\sim\text{7.6}}$/EW$_{z\sim\text{2-6}}$), and infer an IGM HI fraction ($X_{\text{HI}}$) of $49^{+19}_{-19}\%$ at $z\sim7.6$, which is lower in modest tension ($>$1$\sigma$) with recent measurements at $z \sim$ 7.6. The spatial distribution of the detected Ly$\alpha$ emitters implies the presence of a potential highly ionized region at $z\sim7.55$ which hosts four Ly$\alpha$ emitters within a $\sim$ 40 cMpc spatial separation. The prominence of this ionized region in our dataset could explain our lower inferred value of $X_{\text{HI}}$, though our analysis is also sensitive to the chosen reference Ly$\alpha$ EW distribution values and reionization models.
Comment: Published 2020 November 27 in ApJ. Changed to match the published version (HI fraction & Figure 12 updated and other minor changes). 27 pages, 15 figures, 3 tables