The classical test for effects of dust on the overall light emerging from galaxies traces back at le

The classical test for effects of dust on the overall light emerging from galaxies traces back at le

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The classical test for effects of dust on the overall light emerging from galaxies traces back at least to Holmberg's 1958 paper, using the surface-brightness versus inclination test. For transparent galaxies, the surface brightness should vary with apparent axial ratio a/b , since the same light is concentrated to a smaller area, while if galaxies are opaque and we see only a thin skin, the mean surface brightness will be constant with inclination. Holmberg came to the reassuring conclusion that dust effects for global light were minor (though they are clearly important in some regions, as is obvious from so many images). Various correction schemes based on projected axial ratio and Hubble type were used for catalogs and correcting the Tully- Fisher relation. However, two challenges to this view arose about a decade ago. Theoretically, Mike Disney and collaborators (1993 MNRAS 260, 491) presented radiative-transfer models showing that multiple arrangements of stars and dust, some with very large overall extinction, could mimic the observed surface brightness and colors of galaxies, taking into account that strongly obscured stars don't contribute much to the overall intensity. This gives the apparent paradox (chuckled over by Witt, Thronson, & Capuano 1992 ApJ 393, 611) that dusty galaxies can be quite blue. Observationally, Valentijn (1990 Nature 346, 153) analyzed surface
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