Lymanαline and continuum radiative transfer in a clumpy interstellar medium

Abstract

Aims. Studying 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 study 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), and 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, entailing 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 (EWint(Lyα)) for nearly all clumpy ISM configurations 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 > 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 Lyα line profile generally shows no velocity shift and little asymmetry. Otherwise the Lyα line profile is very similar to that 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.

The Lyman alpha reference sample

“…On the one hand, it would be very challenging to apply such velocity dispersions inside pure homogeneous shells of HI and dust (in reality, ∆v exp indicates that the ISM of Mrk 1486 is clumpy, each clump moving outward at a slightly different velocity). On the other hand, our work presented in Duval et al (2014) showed that the effect produced by ∆v exp on the emerging Lyα line profile is very weak and not dominant compared to the one produced by the mean ISM expansion velocity v exp . Throughout our analysis we therefore only considered the mean expansion velocity v exp at which most of the gas is outflowing in each component of our 3D geometry.…”

“…This low uncertainty on v exp is explained by the relatively high sensitivity of the output line profile to the mean expansion velocity v exp of the shells (Verhamme et al 2006;Duval et al 2014). We illustrate in Fig.…”

“…As shown in Verhamme et al (2006) and Duval et al (2014), the mean expansion velocity v exp of an ISM plays the most important role in the formation of the observed Lyα line profile of a galaxy. In particular, the observed wavelength of some redshifted bumps on top of an asymmetric Lyα line prove to be good tracers of the mean ISM expansion velocity v exp , such as the backscattering redshifted peak, as already pointed out by Verhamme et al (2006) for expanding shells 8 .…”

“…The sensitivity of the Lyα line profile to the gas distribution in the ISM of galaxies (Giavalisco et al 1996;Hansen & Oh 2006;Laursen et al 2013;Duval et al 2014;Fig. 17.…”

“…Verhamme et al 2015) enables us to use the Lyα line of Mrk 1486 as a probe of the clumpiness of the HI distribution inside the disk and outflowing halos. As shown in Duval et al (2014), a clumpy distribution of HI and dust can affect the Lyα line profile in three different ways:…”