2025MNRAS.542..104H

Using galaxy Ly$\alpha$ emission to probe reionization relies on establishing baseline expectations for its detectability in the absence of attenuation by neutral gas in the intergalactic medium (IGM). Towards this end, the growing numbers of $z \sim 5$-6 star-forming galaxies spectroscopically selected by James Webb Space Telescope (JWST) provide an ideal sample for determining how Ly$\alpha$ emission depends on galaxy properties and environment after reionization has largely completed. In this study, we use Keck Low-Resolution Imaging Spectrograph (LRIS) to measure the Ly$\alpha$ emission of 46 JWST-selected [O III]-emitting galaxies over $5.3 \lesssim z \lesssim 6.2$ in the foreground of the ultraluminous quasar J0100 + 2802. Overall, we find that the fraction of galaxies detected in Ly$\alpha$ emission is consistent with previous works; however, the fraction also varies with environment. Most notably, we find an apparent deficit of Ly$\alpha$ in the largest group in our sample, at $z \simeq 6.19$, which falls within the redshift range of the quasar's highly ionized proximity zone. We speculate that the Ly$\alpha$ emission from this group may be partly scattered by a foreground neutral island. In contrast, we detect a high rate of Ly$\alpha$ emission in two groups at $z \simeq 5.73$ and $\simeq 5.78$. These groups may be part of a structure that is extended along the line of sight, enhancing the transmission of Ly$\alpha$ emission. While our sample size is limited, our results suggest that environment may play a significant role in the detectability of galaxy Ly$\alpha$ emission even as late as $z \sim 6$.