Lyman α line and continuum radiative transfer in a clumpy interstellar medium

Aims. We study the effects of an inhomogeneous interstellar medium (ISM) on the strength and the shape of the Lyman alpha (Ly alpha) line in starburst galaxies. Methods. Using our 3D Monte Carlo Ly alpha radiation transfer code, we have studied the radiative transfer of Ly alpha, UV, and optical continuum photons in homogeneous and clumpy shells of neutral hydrogen and dust surrounding a central source. Our simulations predict the Ly alpha and continuum escape fraction, the Ly alpha equivalent width EW(Ly alpha), the Ly alpha line profile, and their dependence on the geometry of the gas distribution and the main input physical parameters. Results. The ISM clumpiness is found to have a strong impact on the Ly alpha line radiative transfer, leading to a strong dependence of the emergent features of the Ly alpha line (escape fraction, EW(Ly alpha)) on the ISM morphology. Although a clumpy and dusty ISM appears more transparent to radiation (both line and continuum) compared to an equivalent homogeneous ISM of equal dust optical depth, we find that the Ly alpha photons are, in general, still more attenuated than UV continuum radiation. As a consequence, the observed equivalent width of the Ly alpha line (EWobs(Ly alpha)) is lower than the intrinsic one (EWint(Ly alpha)) for nearly all clumpy ISM configurations being considered. There are, however, special conditions under which Ly alpha photons escape more easily than the continuum, resulting in an enhanced EWobs(Ly alpha). The requirement for this to happen is that the ISM is almost static (galactic outflows <= 200 km s(-1)), extremely clumpy (with density contrasts >10(7) in HI between clumps and the interclump medium), and very dusty (E(B - V) > 0.30). When these conditions are fulfilled, the emergent Ly alpha line profile generally shows no velocity shift and little asymmetry. Otherwise, the Ly alpha line profile is very similar to the one expected for homogeneous media. Conclusions. Given the asymmetry and velocity shifts generally observed in star-forming galaxies with Ly alpha emission, we therefore conclude that clumping is unlikely to significantly enhance their relative Ly alpha/UV transmission.