CRASHα: coupling continuum and line radiative transfer

In this paper we present CRASH alpha, the first radiative transfer code for cosmological application that follows the parallel propagation of Ly alpha and ionizing photons. CRASH alpha is a version of the continuum radiative transfer code CRASH with a new algorithm to follow the propagation of Ly alpha photons through a gas configuration whose ionization structure is evolving. The implementation introduces the time evolution for Ly alpha photons (a feature commonly neglected in line radiative transfer codes) and, to reduce the computational time needed to follow each scattering, adopts a statistical approach to the Ly alpha treatment by making extensive use of pre-compiled tables. These tables describe the physical characteristics of a photon escaping from a gas cell where it was trapped by scattering as a function of the gas temperature/density and of the incoming photon frequency. With this statistical approach we experience a drastic increase of the computational speed and, at the same time, an excellent agreement with the full Ly alpha radiative transfer computations of the code MCLY alpha. We find that the emerging spectra keep memory of the ionization history which generates a given ionization configuration of the gas and, to properly account for this effect, a self-consistent joint evolution of line and ionizing continuum radiation as implemented in CRASH alpha is necessary. A comparison between the results from our code and from Ly alpha scattering alone on a fixed HI density field shows that the extent of the difference between the emerging spectra depends on the particular configuration considered, but it can be substantial and can thus affect the physical interpretation of the problem at hand. These differences should furthermore be taken into account when computing the impact of the Ly alpha radiation on e. g. the observability of the 21-cm line from neutral hydrogen at epochs preceding complete reionization.