LIFTING THE DUSTY VEIL WITH NEAR- AND MID-INFRARED PHOTOMETRY. II. A LARGE-SCALE STUDY OF THE GALACTIC INFRARED EXTINCTION LAW
- Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325 (United States)
We combine near-infrared (Two Micron All Sky Survey) and mid-infrared (Spitzer-IRAC) photometry to characterize the IR extinction law (1.2-8 mum) over nearly 150 deg. of contiguous Milky Way midplane longitude. The relative extinctions in five passbands across these wavelength and longitude ranges are derived by calculating color excess ratios for G and K giant red clump stars in contiguous midplane regions and deriving the wavelength dependence of extinction in each one. Strong, monotonic variations in the extinction law shape are found as a function of angle from the Galactic center, symmetric on either side of it. These longitudinal variations persist even when dense interstellar regions, known a priori to have a shallower extinction curve, are removed. The increasingly steep extinction curves toward the outer Galaxy indicate a steady decrease in the absolute-to-selective extinction ratio (R{sub V} ) and in the mean dust grain size at greater Galactocentric angles. We note an increasing strength of the 8 mum extinction inflection at high Galactocentric angles and, using theoretical dust models, show that this behavior is consistent with the trend in R{sub V} . Along several lines of sight where the solution is most feasible, A {sub l}ambda/A{sub K{sub s}} as a function of Galactic radius (R {sub GC}) is estimated and shown to have a Galactic radial dependence. Our analyses suggest that the observed relationship between extinction curve shape and Galactic longitude is due to an intrinsic dependence of the extinction law on Galactocentric radius.
- OSTI ID:
- 21389270
- Journal Information:
- Astrophysical Journal, Vol. 707, Issue 1; Other Information: DOI: 10.1088/0004-637X/707/1/510; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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