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Title:
Giant Molecular Clouds as Dark Nebulae at 2.4 Microns
Authors:
Dame, T. M.; Fazio, G. G.; Kent, S.; Thaddeus, P.
Affiliation:
AA(Harvard-Smithsonian Center for Astrophysics), AB(Harvard-Smithsonian Center for Astrophysics), AC(Harvard-Smithsonian Center for Astrophysics), AD(Harvard-Smithsonian Center for Astrophysics)
Publication:
Bull. American Astronomical Society, 21, 1181
Publication Date:
09/1989
Origin:
ADS
Bibliographic Code:
1989BAAS...21.1181D

Abstract

The Infrared Telescope flown aboard the Space Shuttle as part of the Spacelab-2 mission surveyed ~75% of the Galactic plane at 2.4 micron with an angular resolution of ~1 deg (Koch et al. 1988, Astrophys. Lett. and Comm., 27, 211). A preliminary comparison of this survey with the Galactic CO map of Dame et al. (1987, Ap. J., 322, 706) reveals a striking anticorrelation toward the inner Galaxy, the CO peaks corresponding to 2.4 micron holes and vice versa. Since the 2.4 micron Galactic emission probably comes mainly from late-type stars, this anticorrelation is best explained as absorption of near- infrared light by dust in the large molecular clouds in the inner spiral arms of the Galaxy -- these clouds, in other words, are so massive that they are dark nebulae at 2.4 micron. Just as star counts have been used to determine the N(H2)/WCO ratio in small, nearby molecular clouds and to provide information on cloud distances, so the infrared absorption determined from the 2.4 micron data may be useful, both to constrain the N(H2)/Wco ratio and to help resolve the kinematic distance ambiguity for large molecular complexes in the inner Galaxy. Guided by previous analyses of the gas distribution in the inner Galaxy and the distributions of late-type stars in external spirals, we are developing a detailed model of the Galactic distributions of gas and late-type stars which can be fit to observed 21 cm, CO, and 2.4 micron maps. In a preliminary model of this sort, we find that the main features of the 2.4 micron longitude profile in the first quadrant can be reproduced using standard values for the ratios N(H2)/Wco, N(H)/Av, and A(2.4micron)/Av, and a molecular cloud distribution similar to that derived by Dame et al. (1986, Ap. J., 305, 892).

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