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Session 22 - Disk Galaxies.
Oral session, Monday, January 13
We present a quantitative investigation of the effects of dust on galaxy colors and observed morphology using BVRIJHK surface photometry of 15 highly inclined, Sab-Sc spirals. A comparison of the optical/IR colors across galaxy dust lanes to predictions from radiative transfer models reveals that only geometries with a mixture of dust and stars produce the saturation of optical colors observed in the dust lanes. We determine effective attenuation curves for ten galaxies in the sample and find that eight of these curves are grayer than the Milky Way extinction law. This indicates that clumpiness in the dust and/or scattering play an important role in determining the reddening in these systems; simple models that neglect these effects produce attenuation curves similar to the Galactic law. The total optical depth inferred from the dust lane reddening is highly dependent on the adopted dust geometry
We also investigate the impact of dust on apparent bulge-to- disk ratio (B/D), which is the primary indicator of Hubble type in highly inclined spirals where the arms are not clearly visible. Some galaxies in our sample show no trend of B/D with wavelength over the range from B to K, while others show a slight increase in B/D at longer wavelengths. This increase is most probably due to the relative dominance of young blue stars in the disk and old red stars in the bulge. If the disk contribution to the total light were significantly decreased by dust, one might expect that B/D would increase at short wavelengths. However, the dominance of red stars in the bulge may compensate for extinction in the disk because the bulge will also emit less light at short wavelengths. We attempt to disentangle these effects by measuring B/D on simulated galaxy images created with known bulge and disk light distributions and a model of radiative transfer in disk galaxies.
Program listing for Monday