Galaxy Protoclusters as Drivers of Cosmic Reionization: I. Bubble Overlap at Redshift z ∼ 7 in LAGER-z7OD1

Abstract Since the launch of JWST, the sample size of reionization-era Ly α emitters (LAEs) has been steadily growing; yet inferences about the neutral hydrogen fraction in the intergalactic medium exhibit increasing variance at redshift z ≈ 7, possibly indicating significant field-to-field fluctuations in the progression of cosmic reionization. In this paper, we present new JWST/NIRSpec and Keck/LRIS spectra of nine LAEs in the redshift z ∼ 7 protocluster LAGER-z7OD1. Measurements of Ly α transmission and Ly α velocity offset along multiple sight lines map the Ly α damping wing optical depth across the galaxy overdensity. In the standard context of inside-out ionization, we estimate the radii of ionized bubbles, R i min = 0.07 − 0.69 Mpc (physical), based on the distance from each LAE to the first neutral patch along the sight line. The resulting 3D topology reveals three distinct subclusters where the ionized bubbles are approaching overlap. Five of the nine LAEs plausibly ionized their bubbles, where a few bursts of star formation and a modest escape fraction are sufficient. We demonstrate, however, that the actual ionized volumes are likely larger, at least R i i s m = 0.42–1.29 Mpc (physical), based on an empirical model for interstellar attenuation of Ly α . Modeling galactic attenuation of Ly α significantly increases the inferred intergalactic transmission (thus enlarging the ionized path length). The error bars on the reddening correction allow fully overlapping bubbles, and our results are consistent with accelerated reionization in the protocluster.