Dust Composition, Energetics, and Morphology of the Galactic Center

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Abstract
Spectra at 16-45 μm of several regions within the central 80'' of the Galaxy have been obtained at 20'' resolution using the Goddard Cryogenic Grating Spectrometer No. 2 on the Kuiper Airborne Observatory. A broad band of excess emission extending from 24 to 45 μm is present in the spectra at positions covering the "tongue" and the inner edge of the circumnuclear disk. A similar dust emission feature has been observed in some carbon-rich evolved stars and in a nitrogen-rich evolved massive star. The observations reported here are the first detection of this dust emission feature in the interstellar medium. We find that the dust properties in the "tongue" and the inner edge of the circumnuclear disk are different from the dust in the cavity, which shows no evidence for the ~30 μm feature in its spectrum. The relation between the tongue feature and the inner disk has been a subject of considerable debate, with suggestions that the tongue material is either falling in or has been expelled from the Galactic center. Our observations suggest that if there is a physical connection between the two regions, then the tongue material is more likely falling in toward the Galactic center. Two kinds of dust components and corresponding heating sources are argued to be responsible for the dust emission in the Galactic center: (1) the hot dust component (140-300 K) heated by IRS sources that are distributed throughout the cavity; and (2) the warm dust component (60-90 K) heated by a He I emission-line star cluster that give rise to a local interstellar radiation field that heats the dust in the central several parsecs. We find a lower limit of ~1 × 106 L☉ for the He I emission-line star cluster. Finally, we find that the line-of-sight extinction across the cavity is not uniform. It is diminished and roughly constant in the northeastern side of the cavity but increases to the southwestern side. Based on this result, together with the 1995 study by Zylka et al. who found that there is a cold dust component along the line of sight to the central 30'' of the Galaxy, we propose that there is a cool dust envelope with nonuniform distribution covering the cavity of the Galactic center.

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