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 capable of mapping the evolution of large-scale structure during the Epoch of Reionization (EoR). Lyman-alpha (Lyα) 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 Lyα 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 Lyα 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 Lyα fluctuation power spectrum at smaller scales ($k \gtrsim 2\times 10^{-2}\, \rm {arcsec}^{-1}$) steepens toward lower redshift, and that our emission-only Lyα power spectrum lies above the SPHEREx sensitivity, whereas the absorption-included signal is ~ 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 ∈ [5.5, 6.5]. We identify limitations in our analysis and propose next steps, including incorporating the effects of resonant Lyα scattering and line interlopers, as well as larger simulation volumes.