The <scp>thesan</scp> project: Lyman- <i>α</i> intensity mapping of cosmic reionization

ABSTRACT Line intensity mapping (LIM) has garnered attention as a powerful cosmological probe, with current-generation instruments such as SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer) capable of mapping the evolution of large-scale structure during the epoch of reionization (EoR). Lyman-alpha (Ly$\alpha$) emission in the EoR is strongly shaped by resonant absorption from neutral hydrogen in the diffuse intergalactic medium (IGM), which transforms galactic sources into a low surface-brightness background. In this work, we leverage the state-of-the-art thesan cosmological simulations to produce high-resolution theoretical predictions for future Ly$\alpha$ LIM studies, constructing continuous light-cones for line-of-sight cosmological integrations. We assess the contributions of recombination, collisional excitation, and unresolved H ii regions to the total Ly$\alpha$ spectral intensity. In addition, we explore the IGM in absorption at different redshifts using damping wing analysis. We produce channel maps exploring spatial fluctuations across redshift bands probe-able by LIM instruments. We find that the slope of the absorption-included Ly$\alpha$ fluctuation power spectrum at smaller scales ($k \gtrsim 2\times 10^{-2}\, \rm {arcsec}^{-1}$) steepens toward lower redshift, and that our emission-only Ly$\alpha$ power spectrum lies above the SPHEREx sensitivity, whereas the absorption-included signal is $\sim \,$7–8 orders of magnitude lower – providing a conservative lower limit on inhomogeneity signatures and highlighting the importance of including resonant scattering in the future. We also find that including outflows in a simple toy model boosts power by four orders of magnitude for a channel map spanning $z\in [5.5,6.5]$. We identify limitations in our analysis and propose next steps, including incorporating the effects of resonant Ly$\alpha$ scattering and line interlopers, as well as larger simulation volumes.