Scattering of Ly{\alpha} Photons through the Reionizing Intergalactic Medium: I. Spectral Energy Distribution

TL;DR

This paper develops a Monte Carlo simulation to study how Lyα photons scatter through the intergalactic medium during reionization, revealing observable spectral and spatial features that inform about early galaxies and IGM properties.

Contribution

It introduces a new Monte Carlo algorithm for modeling Lyα photon scattering in the IGM and applies it to simulated reionization scenarios, providing insights into observable signatures.

Findings

Scattered Lyα light can produce detectable spectral imprints.

Photon scattering causes both reddening and blueshifting effects.

The ionization state influences the scattered light's spectral and spatial characteristics.

Abstract

During reionization, a fraction of galactic Ly$\alpha$ emission is scattered in the intergalactic medium (IGM) and appears as a diffuse light extending megaparsecs from the source. We investigate how to probe the properties of the early galaxies and their surrounding IGM using this scattered light. We create a Monte Carlo algorithm to track individual photons and reproduce several test cases from previous literature. Then, we run our code on the simulated IGM of the CoDaII simulation. We find that the scattered light can leave an observable imprint on the emergent spectrum if collected over several square arcminutes. Scattering can redden the emission by increasing the path lengths of photons, but it can also make the photons bluer by upscattering them according to the peculiar motion of the scatterer. The photons emitted on the far blue side of the resonance appear more extended in both frequency and space compared to those emitted near the resonance. This provides a discriminating feature for the blueward emission, which cannot be constrained from the unscattered light coming directly from the source. The ionization state of the IGM also affects the scattered light spectrum. When the source is in a small HII region, the emission goes through more scatterings in the surrounding HI region regardless of the initial frequency and ends up more redshifted and spatially extended. This can result in a weakening of the scattered light toward high $z$ during reionization. Our results provide a framework for interpreting the scattered light to be measured by high-$z$ integral-field-unit surveys.