The galaxies' energy balance problem solved

Abstract

We attempt to resolve the long-standing energy balance problem encountered by Radiative Transfer (RT) models, particularly in edge-on galaxies by incorporating a treatment for the clumpy structure of the Interstellar Medium (ISM). A subgrid approach is adopted, treating the quiescent dust clouds as pseudo-dust grains, with equivalent optical and thermal emission properties. Deriving key quantities such as the absorption, scattering and extinction cross-sections enables the virtualisation of the macroscopic clump into a microscopic pseudo-grain that can be included alongside the existing dust model constituents. The addition of the pseudo-grain results in a flatter extinction curve. A library of clump emission spectral energy distributions (SEDs) is constructed for radiation fields of various colours and intensities. The new clumpy model is applied to the edge-on galaxy NGC 891 and, for the first time, is able to achieve a good energy balance, simultaneously fitting both the submm and Near Infrared (NIR) data. The clumpy model is further applied to a small sample of seven galaxies of various inclinations, and the results are compared with those from the purely diffuse models. The clumpy models are characterised by a reduction in dust opacity, and therefore attenuation, compared to their purely diffuse counterparts. Thus, the maximum face-on optical depth in the $B$-band, ${\mathrm max}(τ^{\mathrm f}_{\mathrm B})$, derived from the clumpy models is found to be lower by factors ranging from 1.3 to 2.8. Of the seven galaxies, two are found to be optically thick in their centres, two are found to be moderately optically thick, and three are found to be optically thin.