The Impact of Galaxy Overdensities and Ionized Bubbles on Ly$\alpha$ Emission at $z\sim7.0-8.5$

Abstract

Ly$\alpha$ 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 $z\simeq 7$, assuming neutral hydrogen fractions of the intergalactic medium in the range $\overline{x}_{\rm HI}$=0.5-0.7. Here we investigate this picture using JWST to characterize the environment and Ly$\alpha$ emission of 292 galaxies at $7.0<z<8.5$ across 5 fields spanning a comoving volume of $1.3\times10^6$ Mpc$^3$. If the reionization predictions are correct, we should see overdensities and strong Ly$\alpha$ emission clustered in redshift windows of d$z=0.04-0.08$ and angular scales of 5-11 arcmin. We detect Ly$\alpha$ emission in 36 out of 292 galaxies, including nine new Ly$\alpha$ detections, two of which (in the UDS field) show extremely large equivalent widths (EW = $200_{-78}^{+50}$ A and $284_{-75}^{+56}$ A). We identify 13 significant (4-11$\times$) galaxy overdensities using redshifts from NIRCam grism and NIRSpec. Strong Ly$\alpha$ 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$\alpha$ emitters span typical line-of-sight distances (d$z\sim 0.14$) and angular scales ($\sim 8$ arcmin) that are comparable to the predicted bubble sizes at $z\simeq 7$. We discuss evidence that the EGS is mostly ionized along a 24 pMpc sightline at $z\simeq 7.0-7.6$, based on the presence of 3 overdense structures and 10 Ly$\alpha$ emitters in this volume, and find such a large ionized region would pose tension with standard reionization models.