DIAGNOSTICS OF DUST CONTENT IN SPIRAL GALAXIES - NUMERICAL SIMULATIONS OF RADIATIVE-TRANSFER

In order to find the best observable diagnostics for the amount of int ernal extinction within spiral galaxies, we have constructed realistic models for disk galaxies with immersed dust layers. The radiative tra nsfer including both scattering and absorption has been computed for a range of model galaxies in various orientations. Standard galaxy surf ace photometry techniques were then applied to the numerical data to i llustrate how different observables such as total magnitude, color and luminosity distribution behave under given conditions of dust distrib ution. This work reveals a set of superior diagnostics for the dust in the disk. These include not only the integrated parameters, but also the apparent disk structural parameters, the amplitude of the asymmetr y between the near and far sides of the galaxy as divided by the appar ent major axis and their dependence on the orientation of the galaxy w ith respect to the observer. Combining the above diagnostics with our impressions of real galaxies, we arrive at the qualitative conclusion that galaxy disks are generally optically thin. Quantitative conclusio ns will appear in subsequent work.