The Incidence of Large Ionized Bubbles at Redshift 13

Abstract

Ionized bubbles around the first galaxies link early galaxy growth, ionizing photon escape, intergalactic-medium topology, Lyα visibility, 21 cm structure, and the timing of reionization. With JWST now constraining both the abundance of luminous galaxies at $z\gtrsim 10$ and rare Ly$α$ emitters deep in the neutral era, it is timely to ask how often galaxy populations produce large ionized environments. We model the incidence of galaxy-driven ionized bubbles at $z\approx 13$ using JWST UV luminosity functions, taking the Ly$α$ source reported by Witstok et al. (2025) as a benchmark for the relevant bubble scale. We quantify the incidence of regions with comoving radius $R\ge 2.5$ cMpc through the sky surface density $Σ_{\ge 2.5}$ at $z\approx 13$. For our fiducial case (UVLF from Donnan et al. 2024 with $f_{\mathrm{esc}}=0.2$, $\logξ_{\mathrm{ion}}=25.5$, $f_{\mathrm{duty}}=1$, and $C=3$), we find $Σ_{\ge 2.5}\simeq 1.33\times 10^{-2}$ arcmin$^{-2}$ per $Δz=1$. Because bubbles are treated as independent spheres with random source positions and no union of overlaps, this is a conservative baseline for the abundance of connected ionized environments. We conclude that Witstok-sized regions are plausible in UVLF-calibrated galaxy-driven models. This is a population-level statement, however, and the specific Witstok source may still require unusual effective ionizing efficiency, recent fading or burstiness, or a non-stellar ionizing contribution.