AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 71. Between the Stars II: The ISM, Galactic and Extragalactic
Display, Wednesday, June 2, 1999, 10:00am-6:30pm, Southwest Exhibit Hall

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[71.21] Reddening and Attenuation by Dust in Galaxies with a Clumpy ISM

A.N. Witt (U. Toledo), K.D. Gordon (LSU)

The UV/optical flux and SED of galaxies are strongly affected by the wavelength-dependent attenuation of light by dust internal to galaxies. One important characteristic of such dust is its inhomogeneous, clumpy distribution. We report on new radiative transfer models applicable to galaxies which include a locally random, two-phase ISM with scattering dust. We investigated the effects of varying the geometries of relative star/dust distributions and the effects of dust type (MW dust and SMC dust), in addition to comparing the effects of clumpy and homogeneous dust distributions. Our models cover the wavelength range 1000 Å to 30,000 Å.

Increasing column densities of clumpy dust lead to rapidly saturating reddening at low values of E(B-V), resulting in an increasingly gray attenuation, especially in the UV. The determination of an attenuation correction factor in the UV, of major significance for the analysis of the SEDs of UV-selected starburst galaxies in the nearby and distant Universe (i.e. Lyman-break galaxies) from UV spectral characteristics alone is highly geometry- and dust-type-dependent. We show that the spectral characteristics of a sample of UV-selected, nearby starburst galaxies are reproduced most easily with a shell geometry containing clumpy dust with SMC-type extinction and scattering characteristics. The complete set of models is available in electronic form from the authors.

This work has been supported by grants from NASA, which we acknowledge with gratitude.

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