Lyman-alpha radiative transfer during the Epoch of Reionization: contribution to 21-cm signal fluctuations

TL;DR

This paper presents a new Monte-Carlo radiative transfer simulation of Ly-alpha photons during the Epoch of Reionization, highlighting the importance of wing scatterings and anisotropic large-scale structures for 21-cm signal fluctuations.

Contribution

It introduces an improved Ly-alpha radiative transfer model that accounts for wing scatterings and anisotropic structures, enhancing predictions of 21-cm signal fluctuations during reionization.

Findings

Wing scatterings steepen P_alpha radial profiles.

Large-scale filament structures cause anisotropic scattering effects.

Fluctuation contrast is reinforced by wing scatterings.

Abstract

During the epoch of reionization, Ly-alpha photons emitted by the first stars can couple the neutral hydrogen spin temperature to the kinetic gas temperature, providing the opportunity to observe the gas in emission or absorption in the 21-cm line. Given the bright foregrounds, it is of prime importance to determine precisely the fluctuations signature of the signal, to be able to extract it by its correlation power. LICORICE is a Monte-Carlo radiative transfer code, coupled to the dynamics via an adaptative Tree-SPH code. We present here the Ly-alpha part of the implementation, and validate it through three classical tests. Contrary to previous works, we do not assume that P_alpha, the number of scatterings of Ly-alpha photons per atom per second, is proportional to the Ly-alpha background flux, but take into account the scatterings in the Ly-alpha line wings. The latter have the effect to steepen the radial profile of P_alpha around each source, and re-inforce the contrast of the fluctuations. In the particular geometry of cosmic filaments of baryonic matter, Ly-alpha photons are scattered out of the filament, and the large scale structure of P_alpha is significantly anisotropic. This could have strong implications for the possible detection of the 21-cm signal.