Multiple scattering in clumpy media. II. Galactic environments

We present and discuss the results of new multiple-scattering radiative tra nsfer calculations for three representative types of galactic environments, filled with either homogeneous or two-phase clumpy dust distributions. Ext inction and scattering properties for two types of interstellar dust, simil ar to those found in the average diffuse medium of the Milky Way (MW) and t he Bar of the Small Magellanic Cloud (SMC), are considered. The wavelength coverage extends from 1000 to 30000 Angstrom with particular emphasis on th e rest-frame UV. This makes these models especially applicable to starburst galaxies and Lyman-break galaxy samples. The examination of the models con centrates on the study of UV/visual/ near-IR reddening effects, the wavelen gth dependence of attenuation, and on the changes that arise from the trans ition from homogeneous to clumpy dust distributions in different star/dust geometries. Embedded dust, especially when clumpy, leads to saturation at f airly low reddening values with correspondingly gray attenuation functions. This makes the assessment of the attenuation of the far-UV flux from starb urst galaxies difficult, if only UV/visual/near-IR data are available. Exis ting data for UV-selected starburst galaxies indicate a range of UV attenua tion factors of 0-150. Our models reproduce the "Calzetti Attenuation Law," provided one adopts SMC-type dust and a clumpy shell-type dust distributio n surrounding the starbursts. The average far-UV attenuation factor for the Calzetti sample is found to be 7.4. The only relatively reliable measure f or 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 ga laxies.