The THESAN project: Lyman-alpha emitters as probes of ionized bubble sizes

We use the THESAN radiation-hydrodynamics simulations to investigate how Lyman- α emitters (LAEs) trace ionized bubble sizes during the Epoch of Reionization. We generate realistic LAE catalogs by combining accurate intrinsic Ly α production and intergalactic transmission with an empirical model for dust absorption and gas outflows. By calibrating to observationally-constrained Ly α luminosity functions, we reproduce the rapid decline in Ly α visibility toward higher redshifts while revealing mild tensions in LAE fractions near the end of reionization. Before the midpoint of reionization, galaxies within larger line-of-sight bubbles ( ≳ 10 cMpc) have higher observed Ly α luminosity and equivalent width (EW), demonstrating that the evolving LAE fraction provides a practical statistical tracer for bubble size. These correlations weaken as percolation progresses and the IGM becomes increasingly ionized. In LAE selected samples with L Ly α > 10 41.5 erg s − 1 , Ly α properties correlate with bubble size more strongly than UV magnitude, especially at z ≳ 7 . This simulation-based framework maps LAE selections to bubble-size statistics, clarifies biases in more idealized models, and will supply public catalogs to interpret current and forthcoming JWST and narrow-band LAE surveys in terms of the evolving topology of reionization.