The DIRTY model. II. Self-consistent treatment of dust heating and emission in a three-dimensional radiative transfer code

In this paper and a companion paper we present the DIRTY model, a Monte Carlo radiative transfer code, self-consistently including dust heating and emission and accounting for the effects of the transient heating of small grains. The code is completely general : the density structure of the dust, the number and type of heating sources, and their geometric configurations can be specified arbitrarily within the model space. Source photons are tracked through the scattering and absorbing medium using Monte Carlo techniques, and the effects of multiple scattering are included. The dust-scattering, -absorbing, and -emitting properties are calculated from realistic dust models derived by fitting observed extinction curves in Local Group galaxies, including the Magellanic Clouds and the Milky Way. The dust temperature and the emitted dust spectrum are calculated self-consistently from the absorbed energy, including the effects of temperature fluctuations in small grains. Dust self-absorption is also accounted for, allowing the treatment of high optical depths, by treating photons emitted by the dust as an additional heating source and adopting an iterative radiative transfer scheme. As an illustrative case, we apply the DIRTY radiative transfer code to starburst galaxies wherein the heating sources are derived from stellar evolutionary synthesis models. Within the context of the starburst model, we examine the dependence of the ultraviolet to far-infrared spectral energy distribution, dust temperatures, and dust masses predicted by DIRTY on variations of the input parameters.