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

TL;DR

This study uses advanced simulations to show how Lyman-alpha emitters can serve as effective probes for measuring ionized bubble sizes during the Epoch of Reionization, aiding interpretation of observational data.

Contribution

It introduces a simulation-based framework linking LAE properties to ionized bubble sizes, improving understanding of reionization topology and biases in simpler models.

Findings

LAE visibility declines rapidly at higher redshifts.

Larger ionized bubbles correlate with higher Lyα luminosity and EW before reionization midpoint.

Lyα properties correlate more strongly with bubble size than UV magnitude at z ≥ 7.

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.