Radiative Transfer Model of Dust Attenuation Curves in Clumpy, Galactic Environments

The attenuation of starlight by dust in galactic environments is investigated through models of radiative transfer in a spherical, clumpy ISM. Extinction properties for MW, LMC, and SMC dust types are considered. It is illustrated that the attenuation curves are primarily determined by the wavelength dependence of absorption rather than by the underlying extinction (absorption+scattering) curve. Attenuation curves consistent with the "Calzetti attenuation curve" are found by assuming the silicate-carbonaceous dust model for the MW, but with the 2175A absorption bump suppressed or absent. The discrepancy between our results and previous work that claimed the SMC-type dust to be the most likely origin of the Calzetti curve is ascribed to the difference in adopted albedos; this study uses the theoretically calculated albedos whereas the previous ones adopted empirically derived albedos from observations of reflection nebulae. It is also found that the model attenuation curves calculated with the MW dust are well represented by a modified Calzetti curve with a varying slope and UV bump strength. The strong correlation between the slope and UV bump strength, with steeper curves having stronger bumps, as found in star-forming galaxies at 0.5<z<2.0, is well reproduced by our models if the abundance of the UV bump carriers or PAHs is assumed to be 30% or 40% of that of the MW-dust. The trend is explained by radiative transfer effects which lead to shallower attenuation curves with weaker UV bumps as the ISM is more clumpy and dustier. We also argue that at least some of the IUE local starburst galaxies may have a UV bump feature in their attenuation curves, albeit much weaker than that of the MW extinction curve.