THESAN reveals Lyman-alpha secrets of reionization?

THESAN is a large project that uses powerful simulations to study the ultraviolet light left by the early universe. When the universe was young, galaxies emitted Lyman-alpha photons, but the neutral intergalactic medium absorbed them, turning them into a faint background glow. THESAN creates detailed maps of this light, showing how the signal changes with redshift. Researchers measured three main sources: recombination of atoms, collisional excitation, and unresolved HII regions. They also analyzed the intergalactic medium's absorption, finding that the power of light fluctuations becomes steeper at small scales as the universe ages. The emission-only signal is strong enough to be seen by future instruments like SPHEREx, but when absorption is included, the signal drops dramatically—by up to four orders of magnitude. A simple model of galactic winds boosts the signal by the same amount, highlighting the importance of these outflows. THESAN helps us understand how light moves through the early universe and prepares the way for even more precise observations with upcoming telescopes.

THESAN (The Evolution of Structures and reIONization) is a high‑resolution cosmological simulation suite that produces continuous light cones for realistic lines of sight, enabling theoretical predictions for future Lyman‑alpha Line Intensity Mapping (LIM) studies. The simulations incorporate the full physics of the epoch of reionization: star formation, supernova feedback, metal enrichment, and resonant Lyman‑alpha scattering. Using a simulation volume of ~100 Mpc/h per side and a resolution of 512³ cells, the authors compute the total Lyman‑alpha spectral intensity as the sum of three components: recombination (from first‑star gas), collisional excitation (from atom–particle interactions), and unresolved HII regions (from the galaxy formation model). Channel maps in redshift bins (z≈6–10) reveal fluctuations on scales from 1 to 10 Mpc/h. Damping‑wing analysis shows a gradual increase of absorption with redshift, consistent with quasar observations. The emission‑only fluctuation power exceeds SPHEREx sensitivity on scales k>10⁻² 1/arcsec, while including absorption reduces the signal by ~4 orders of magnitude, providing a conservative lower limit for inhomogeneity signatures. A simple toy model adding isotropic winds boosts the signal by four orders of magnitude, indicating that galactic feedback may be crucial for interpreting future LIM data. Current limitations include the lack of full resonant Lyman‑alpha scattering and spectral interlopers (e.g., metal lines). Next steps are to incorporate resonant scattering, expand simulation volumes, and directly compare with ongoing DESI and Euclid mapping data.