Starburst galaxies: Why the Calzetti dust extinction law?

The empirical reddening function for starburst galaxies generated by Calzetti and her coworkers has proved very successful and is now used widely in the observational literature. Despite its success, however, the physical basis for this extinction law, or more correctly, attenuation law remains weak. Here we provide a physical explanation for the Calzetti law based on a turbulent interstellar medium. In essence, this provides a lognormal distribution of column densities, giving a wide range of column densities in the dusty foreground screen. Therefore, extended sources such as starburst regions or H II regions seen through it suffer a point-to-point stochastic extinction and reddening. Regions of high column densities are "black" in the UV but translucent in the IR, which leads to a flatter extinction law and a larger value of the total to selective extinction, R_V. We fit the Calzetti law and infer that the variance σ of the lognormal distribution lies in the range 0.6 ≤ σ ≤ 2.2. The absolute to selective extinction R _V is found to be in the range 4.3-5.2 consistent with R_V = 4.05 ± 0.80 of the Calzetti law.