The Impact of Galaxy Overdensities and Ionized Bubbles on Ly <i>α</i> Emission at <i>z</i>  ∼ 7.0–8.5

Abstract Ly α spectroscopy with JWST is opening a new window on the sizes of ionized bubbles through the reionization epoch. Theoretical expectations suggest typical bubble radii should be 0.6–1.5 pMpc at z ≃ 7, assuming neutral hydrogen fractions of the intergalactic medium in the range x ¯ HI = 0.5–0.7. Here, we investigate this picture using JWST to characterize the environment and Ly α emission of 292 galaxies at 7.0 < z < 8.5 across five fields spanning a comoving volume of 1.3 × 10 6 Mpc 3 . If the reionization predictions are correct, we should see overdensities and strong Ly α emission clustered in redshift windows of d z = 0.04–0.08 and angular scales of 5′–11′. We detect Ly α emission in 36 out of 292 galaxies, including nine new Ly α detections, two of which (in the UltraDeep Survey field) show extremely large equivalent widths (EW = 20 0 − 78 + 50 Å and 28 4 − 75 + 56 Å). We identify 13 significant (4–11×) galaxy overdensities using redshifts from NIRCam grism and NIRSpec. Strong Ly α emitters are almost uniformly found in the overdensities, with nearly all located between the center and back of the structures. The overdensities that host the strong Ly α emitters span typical line-of-sight distances ( d z ∼ 0.14) and angular scales (∼8′) that are comparable to the predicted bubble sizes at z ≃ 7. We discuss evidence that the Extended Groth Strip is mostly ionized along a 24 pMpc sight line at z ≃ 7.0–7.6, based on the presence of three overdense structures and 10 Ly α emitters in this volume, and find such a large ionized region would pose tension with standard reionization models.