The Impact of Neutral Intergalactic Gas on Lyα Intensity Mapping during Reionization

Abstract We present the first simulations of the high-redshift Ly α intensity field that account for scattering in the intergalactic medium (IGM). Using a 3D Monte Carlo radiative transfer code, we find that Ly α scattering smooths spatial fluctuations in the Ly α intensity on small scales, and that the spatial dependence of this smoothing depends on the mean neutral fraction of the IGM. Our simulations find a strong effect of reionization on k = 0.1–1 Mpc −1 , with for and for , in contrast to after reionization. At wavenumbers of k > 1 Mpc −1 , we find that the signal is sensitive to the emergent Ly α line profiles from galaxies. We also demonstrate that the cross-correlation between a Ly α intensity map and a future galaxy redshift survey could be detected on large scales by an instrument similar to SPHEREx , and over a wide range of scales by a hypothetical intensity mapping instrument in the vein of the Cosmic Dawn Intensity Mapper ( CDIM ).