Analytical model for scattered Lyα emission in the post-reionization intergalactic medium

Abstract

Lyα intensity mapping is emerging as a probe of faint galaxies and the cosmic web that elude traditional surveys, but resonant Lyα radiative transfer complicates the interpretation of observed data. We develop a fast and accurate analytical prescription for computing the Lyα intensity field on megaparsec scales in the post-reionization universe. Motivated by Monte Carlo radiative-transfer experiments, we exploit the fact that in a highly ionized intergalactic medium with negligible damping-wing opacity, cosmological redshifting quickly drives Lyα photons out of resonance and terminates the scattering process. Photons emitted blueward of line center tend to scatter on a thin, nearly spherical surface of last scattering, with a radius set by the redshifting distance to resonance. We derive closed-form expressions for scattered emissivity and projected surface brightness that depend only on the source spectrum, neutral-hydrogen density, and peculiar-velocity field. Applied to a source in a realistically simulated intergalactic medium at z = 3, the model shows mild discrepancies with Monte Carlo results within a physical megaparsec of the host halo, where gravitational infall redistributes scattered photons, but achieves better than 5% accuracy beyond that distance in the angle-averaged radial surface-brightness profile. This prescription offers a computationally efficient alternative for forward-modeling Lyα intensity maps from cosmological simulations.