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

¹ Department of AstronomyStockholm University, Oscar Klein Center, AlbaNova, 106 91 Stockholm, Sweden
e-mail: fduva@astro.su.se
² Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
³ CNRS, IRAP, 14 avenue E. Belin, 31400 Toulouse, France
⁴ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Cøpenhagen, Denmark

Received: 27 September 2012
Accepted: 10 May 2013

Abstract

Aims. We study the effects of an inhomogeneous interstellar medium (ISM) on the strength and the shape of the Lyman alpha (Lyα) line in starburst galaxies.

Methods. Using our 3D Monte Carlo Lyα radiation transfer code, we have studied the radiative transfer of Lyα, UV, and optical continuum photons in homogeneous and clumpy shells of neutral hydrogen and dust surrounding a central source. Our simulations predict the Lyα and continuum escape fraction, the Lyα equivalent width EW(Lyα), the Lyα 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 Lyα line radiative transfer, leading to a strong dependence of the emergent features of the Lyα line (escape fraction, EW(Lyα)) 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 Lyα photons are, in general, still more attenuated than UV continuum radiation. As a consequence, the observed equivalent width of the Lyα line (EW_obs(Lyα)) is lower than the intrinsic one (EW_int(Lyα)) for nearly all clumpy ISM configurations being considered. There are, however, special conditions under which Lyα photons escape more easily than the continuum, resulting in an enhanced EW_obs(Lyα). The requirement for this to happen is that the ISM is almost static (galactic outflows ≤200 km s^-1), extremely clumpy (with density contrasts >10⁷ in HI between clumps and the interclump medium), and very dusty (E(B − V) > 0.30). When these conditions are fulfilled, the emergent Lyα line profile generally shows no velocity shift and little asymmetry. Otherwise, the Lyα 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 Lyα emission, we therefore conclude that clumping is unlikely to significantly enhance their relative Lyα/UV transmission.