  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)
