The Infrared Spectrum of the Galactic Center and the Composition of Interstellar Dust

Abstract

We have obtained 5-8 mu m spectra of the Galactic center from the Kuiper Airborne Observatory at resolving powers of approximate to 50, approximate to 150, and approximate to 300. These spectra show absorption features at 5.5, 5.8, 6.1, and 6.8 mu m. Together with previously observed features in the 3 mu m region, these features are compared with laboratory spectra of candidate materials. The 3.0 and 6.1 mu m features are due to the OH stretching and bending variations of H2O and are well fitted by water of hydration in silicates (e.g., talc). The 3.0 mu m band is equally well fitted by ice mixtures containing 30% H2O, but such mixtures do not provide a good fit to the observed 6.1 mu m band. The 3.4 and 6.8 mu m features are identified with the CH stretching and deformation modes in CH, and CH, groups in saturated aliphatic hydrocarbons. The 6.1 mu m band shows a short wavelength shoulder centered on 5.8 mu m, attributed to carbonyl (C=O) groups in this interstellar hydrocarbon dust component. Finally, the narrow 5.5 mu m feature is also attributed to carbonyl groups, but in the form of metal carbonyls [e.g., Fe(CO)(4)]. We have derived column densities and abundances along the line of sight toward the Galactic center for the various identified dust components. This analysis shows that hydrocarbon grains contain only 0.08 of the elemental abundance of C and contribute only a relatively minor fraction (0.1) of the total dust volume. Most of the interstellar dust volume is made up of silicates (approximate to 0.6). Small graphite grains, responsible for the 2200 Angstrom bump, account for 0.07 of the total dust volume. The remaining one-quarter of the interstellar dust volume consists of a material(s) without strong IR absorption features. Likely candidates include large graphite grains, diamonds, or amorphous carbon grains, which all have weak or no IR active modes. Finally, various models for the origin of the hydrocarbon dust component of the interstellar dust are discussed. All of them face some problems in explaining the observations, in particular, the absence of the spectroscopic signature of hydrocarbon grains in sources associated with molecular clouds.