During the epoch of reionization, Ly-α photons emitted by the first stars can couple the neutral hydrogen spin temperature to the kinetic gas temperature, providing an opportunity to observe the gas in emission or absorption in the 21-cm line. Given the bright foregrounds, it is particularly important to determine the fluctuation signature of the signal precisely, so as 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-α part of the implementation and validate it through three classical tests. Unlike previous works, we do not assume that , the number of scatterings of Ly-α photons per atom per second, is proportional to the Ly-α background flux, but take the scatterings in the Ly-α line wings into account. The latter have the effect of steepening the radial profile of around each source, and re-inforce the contrast of the fluctuations. In the particular geometry of cosmic filaments of baryonic matter, photons are scattered out of the filament, and the large-scale structure of is significantly anisotropic. This could have strong implications for the possible detection of the 21-cm signal.
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Semelin et al. (2007) studied this question.