Multiple scattering in clumpy media. II. Galactic environments

We present and discuss the results of new multiple-scattering radiative transfer calculations for three representative types of galactic environments, filled with either homogeneous or two-phase clumpy dust distributions. Extinction and scattering properties for two types of interstellar dust, similar to those found in the average diffuse medium of the Milky Way (MW) and the Bar of the Small Magellanic Cloud (SMC), are considered. The wavelength coverage extends from 1000 to 30000 Angstrom with particular emphasis on the rest-frame UV. This makes these models especially applicable to starburst galaxies and Lyman-break galaxy samples. The examination of the models concentrates on the study of UV/visual/ near-IR reddening effects, the wavelength dependence of attenuation, and on the changes that arise from the transition from homogeneous to clumpy dust distributions in different star/dust geometries. Embedded dust, especially when clumpy, leads to saturation at fairly low reddening values with correspondingly gray attenuation functions. This makes the assessment of the attenuation of the far-UV flux from starburst galaxies difficult, if only UV/visual/near-IR data are available. Existing data for UV-selected starburst galaxies indicate a range of UV attenuation factors of 0-150. Our models reproduce the "Calzetti Attenuation Law," provided one adopts SMC-type dust and a clumpy shell-type dust distribution surrounding the starbursts. The average far-UV attenuation factor for the Calzetti sample is found to be 7.4. The only relatively reliable measure for the UV attenuation factor for single galaxies was found in the ratio of the integrated far-IR flux to the far-UV flux, measured near 1600 Angstrom, requiring the measurement of the entire spectral energy distribution of galaxies.