3D Lyman-alpha radiation transfer. I. Understanding Lyman-alpha line profile morphologies

Using a Monte Carlo technique, we have developed a 3D lyman-alpha radiation transfer code allowing for prescribed arbitrary hydrogen density, ionisation, temperature structures, and dust distribution, and arbitrary velocity fields and UV photon sources. We have examined the lyman-alpha line profiles predicted for several simple geometrical configurations and their dependence on the main input parameters. Overall, we find line profiles reaching from doubly peaked symmetric emission to symmetric Voigt (absorption) in static configurations with increasing dust content, and asymmetric red-(blue-) shifted emission lines with a blue (red) counterpart ranging from absorption to emission (with increasing line/continuum strength) in expanding (infalling) media. The following results are of interest for the interpretation of lya profiles from galaxies. 1/ Standard lya absorption line fitting of global spectra of galaxies may lead to an underestimate of the true hydrogen column density in certain geometrical conditions. 2/ Normal (inverted) P-Cygni like lya profiles can be obtained in expanding (infalling) media from objects without any intrinsic lya emission, as a natural consequence of radiation transfer effects. 3/ The formation and the detailed shape of lya profiles resulting from expanding shells has been thoroughly revised: for sufficiently large column densities, the position of the main lya emission peak is redshifted by twice the expansion velocity.This is in excellent agreement with the observations of z~3 LBGs. This finding indicates also that large scale, fairly symmetric shell structures must be a good description for the outflows in LBGs.(shortened abstract)