A simple model for the absorption of starlight by dust in galaxies

We present a new model for computing the effects of dust on the integrated spectral properties of galaxies, based on an idealized description of the m ain features of the interstellar medium (ISM). The model includes the ioniz ation of H II regions in the interiors of the dense clouds in which stars f orm and the influence of the finite lifetime of these clouds on the absorpt ion of radiation. We compute the production of emission lines and the absor ption of continuum radiation in the H II regions and the subsequent transfe r of line and continuum radiation in the surrounding H I regions and the am bient ISM. This enables us to interpret simultaneously all the observations of an ultraviolet-selected sample of nearby starburst galaxies, including the ratio of far-infrared to ultraviolet luminosities, the ratio of H alpha to H beta luminosities, the H alpha equivalent width, and the ultraviolet spectral slope. We show that the finite lifetime of stellar birth clouds is a key ingredient for resolving an apparent discrepancy between the attenua tion of line and continuum photons in starburst galaxies. In addition, we f ind that an effective absorption curve proportional to lambda(-0.7) reprodu ces the observed relation between the ratio of far-infrared to ultraviolet luminosities and the ultraviolet spectral slope. We interpret this relation most simply as a sequence in the overall dust content of the galaxies. The shallow wavelength dependence of the effective absorption curve is compati ble with the steepness of known extinction curves if the dust has a patchy distribution. In particular, we find that a random distribution of discrete clouds with optical depths similar to those in the Milky Way provides a co nsistent interpretation of all the observations. A noteworthy outcome of ou r detailed analysis is that the observed mean relations for starburst galax ies can be closely approximated by the following simple recipe: use an effe ctive absorption curve proportional to lambda(-0.7) to attenuate the line a nd continuum radiation from each stellar generation, and lower the normaliz ation of the curve, typically by a factor of 3 after 10(7) yr, to account f or the dispersal of the birth clouds. This recipe or our full model for abs orption can be incorporated easily into any population synthesis model.