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Session 48 - Spiral & Field Galaxies.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center
Partially overlapping pairs of galaxies offer a unique opportunity to directly measure dust extinction in the disks of spiral galaxies. When both galaxies are sufficiently symmetric, we can construct e^-\tau maps of the foreground galaxy using symmetry and purely differential photometry. No assumptions about the vertical disk structure or relative distributions of stars and dust are required.
We present results from a B and I survey of some of the most promising pairs for opacity determinations. We find that spiral arms can have high optical depths at any radius, while interarm regions have small optical depths in the outer disks, rising to \tau_B\approx1 within \sim0.4R_25. Scattering effects are deduced to be small. The wavelength dependence of the opacity is flatter (``grayer") than the usual Galactic extinction curve, an indication of clumped extinction. Since most of the extinction is spatially correlated with the bulk of the blue light, this may explain why the surface brightness of spirals is virtually independent of inclination, without requiring high opacities throughout disks, contrary to the usual interpretation.
This research was conducted as part of an NSF Research Experience for Undergraduates program under grant AST-9424226.
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