Reionization in Protocluster Environments at $z>7$ with JWST/NIRSpec

Abstract

Understanding the role of high-redshift protoclusters in cosmic reionization is essential to unveiling the early stages of structure formation. Using deep imaging and spectroscopy from the James Webb Space Telescope (JWST) JADES Deep Survey in GOODS-South, we identify two prominent protoclusters at z>7 and investigate their environmental properties in comparison to field galaxies. Protocluster members exhibit systematically higher ionizing photon production efficiency ($\xi_{\text{ion}}$) and inflated [OIII]/H$\beta$ ratios at fixed UV magnitude or stellar mass, likely driven by young, metal-poor stellar populations and intense star formation. Despite these properties, their Ly$\alpha$ emission is weak or absent, and they show high proximate neutral hydrogen column densities, suggesting insufficient ionizing output to maintain ionized bubbles. We also find that a strong Ly$\alpha$ emitter (LAE), JADES-GS-z7-LA, may lie within the same ionized region as one protocluster. Although their Lyman continuum escape fractions ($f_{\mathrm{esc}} \sim 0.1$) are comparable to those of LAEs, individual protocluster galaxies are faint ($M_{\mathrm{UV}} > -19$) and low-mass ($\log(M_*/M_\odot) \sim 8.5$). The enhanced number density within protoclusters boosts the local UV luminosity density by nearly 1 dex. The surrounding gas remains largely neutral, suggesting that reionization was highly patchy and modulated by environment. The protocluster galaxies likely host ionization-bounded nebulae with holes, suppressing Ly$\alpha$ visibility, in contrast to field galaxies that are more consistent with density-bounded nebulae.