Dust and the transfer of stellar radiation within galaxies

We present and discuss models for the transfer of ultraviolet, visual, and near-infrared radiation within a variety of spherical geometries which are intended to approximate different types of dusty environments within galaxies. Among those objects that our models approximate are normal and "starburst" galaxies, active galactic nuclei and QSOs, colliding systems with a widely dispersed interstellar medium, and elliptical galaxies. Our mathematical technique is a complete spherically symmetric, three-dimensional Monte Carlo simulation which covers a wide range of dust optical depths for 10 photometric bands. We apply our results to several currently fashionable topics in extragalactic astronomy. We emphasize the transfer of stellar radiation, but our results are immediately applicable to other sources of radiation, such as that from nuclear accretion disks or very hot dust emitting in the near- infrared. In addition, although our interest here is galaxies, our models can be directly applied to other objects, such as dusty nebulae. Quantitatively, our results depend sensitively upon adopted geometry, largely due to the important effects of scattering and to the relative fraction of stars that are lightly obscured in each model. Qualitatively, however, some general trends are clear. In the first place, much previous work on the effects of dust on stellar radiation has been simplistic, wrong, and usually both. This statement applies equally forcefully to many past attempts to correct the stellar radiation or broad-band colors for the effects of dust. One calculated result is that "bluing" due to scattering partially compensates for reddening by extinction. This is an effective process that reduces some observational consequences of large amounts of dust so that common techniques for estimating the amount of dust fail by large factors. As one example, stars distributed throughout a dusty environment lead to large ratios of far-infrared-to-visual luminosity without a significant increase in reddening of broadband colors. Furthermore, for one plausible geometry we calculate that the maximum reddening occurs for intermediate optical depths, while both very small and very large amounts of dust produce almost neutral broad-band colors. Similarly, predicted reddening vectors in color-color diagrams are different from those usually chosen to analyze extragalactic photometry. Transfer of radiation through a dusty medium is also able to reproduce some color and surface brightness variations in elliptical galaxies that have usually been attributed to metallicity or population variations.