The reionizing bubble size distribution around galaxies

Lyman-alpha (Ly alpha) emission from galaxies is currently our most promising probe for constraining when and how reionization began, and thus when the first galaxies formed. At z > 7, the majority of galaxies detected with Ly alpha are in candidate overdensities. Here, we quantify the probability of these galaxies residing in large ionized bubbles. We create (1.6 Gpc)(3) intergalactic medium (IGM) simulations: sufficient volume to robustly measure bubble size distributions around UV-bright galaxies and rare overdensities. We find ${M_{\small UV}}\lesssim -16$ galaxies and overdensities are greater than or similar to 10-1000 x more likely to trace ionized bubbles compared to randomly selected positions. The brightest galaxies and strongest overdensities have bubble size distributions with highest characteristic size and least scatter. We compare two models: gradual reionization driven by numerous UV-faint galaxies versus rapid reionization by rarer brighter galaxies, producing larger bubbles at fixed neutral fraction. We demonstrate that recently observed z similar to 7 overdensities are highly likely to trace large ionized bubbles, corroborated by their high Ly alpha detection rates. However, Ly alpha detections at z approximate to 8.7 in EGS and z = 10.6 in GN-z11 are unlikely to trace large bubbles in our fiducial model - 11 and 7 per cent probability of >1 proper Mpc bubbles, respectively. Ly alpha detections at such high redshifts could be explained by: a less neutral IGM than previously expected; larger ionized regions at fixed neutral fraction; or if intrinsic Ly alpha flux is unusually strong in these galaxies. We discuss how to test these scenarios with JWST and prospects for upcoming wide-area surveys to distinguish between reionization models.