The Composition and Distribution of Dust along the Line of Sight toward the Galactic Center

We discuss the composition of dust and ice along the line of sight to theGalactic Center (GC) based on analysis of mid-infrared spectra (2.4-13 micron)from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO).We have analyzed dust absorption features arising in the molecular cloudmaterial and the diffuse interstellar medium along the lines of sight towardSagittarius A* and the Quintuplet sources, GCS3 and GCS4. It is evident fromthe depth of the 3.0 micron H2O and the 4.27 micron CO2 ice features that thereis more molecular cloud material along the line of sight toward Sgr A* thanGCS3 and 4. In fact, Sgr A* has a rich infrared ice spectrum with evidence forthe presence of solid CH4, NH3, and possibly HCOOH. Hydrocarbon dust in the diffuse interstellar medium along the line of sightto the GC is characterized by absorption features centered at 3.4, 6.85, and7.25 micron. Ground-based studies have identified the 3.4 micron feature withaliphatic hydrocarbons, and ISO has given us the first meaningful observationsof the corresponding modes at longer wavelengths. The integrated strengths ofthese three features suggest that hydrogenated amorphous carbon is theircarrier. We attribute an absorption feature centered at 3.28 micron in the GCS3spectrum to the C-H stretch in aromatic hydrocarbons. This feature is notdetected, and its C-C stretch counterpart appears to be weaker, in the SgrA*spectrum. One of the key questions which now arises is whether aromatics are awidespread component of the diffuse interstellar medium, analogous to aliphatichydrocarbons.Comment: 37 pages, including 14 figures and 6 tables. Accepted to Astrophys. J., to appear V. 537, Jul. 10, 200