Fully threaded transport engine: new method for multi-scale radiative transfer

A new, very fast method for 3D radiative transfer on fully threaded grids with arbitrarily high angular resolution is presented. The method uses completely cell-based discretization, and is ideally suited for problems with diffuse background radiation, often encountered in cosmological and star formation models. We find that for accurate statistical study of intergalactic Ly alpha absorption lines, one needs of the order of a few hundred angular discretization elements even for models without radiative feedback from star-forming galaxies.