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

Abstract: We use the THESAN radiation-hydrodynamics simulations to investigate how Lyman-$\alpha$ emitters (LAEs) trace ionized bubble sizes during the Epoch of Reionization. We generate realistic LAE catalogs by combining accurate intrinsic Ly$\alpha$ production and intergalactic transmission with an empirical model for dust absorption and gas outflows. By calibrating to observationally-constrained Ly$\alpha$ luminosity functions, we reproduce the rapid decline in Ly$\alpha$ 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 ($\gtrsim 10$ cMpc) have higher observed Ly$\alpha$ 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 $L_{\text{Ly}\alpha} > 10^{41.5}\ \text{erg s}^{-1}$, Ly$\alpha$ properties correlate with bubble size more strongly than UV magnitude, especially at $z \gtrsim 7$. 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.