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 the Galactic center (GC) based on analysis of mid-infrared spectra (2.4-13 μm) from the Short Wavelength Spectrometer on the Infrared Space Observatory (ISO). We have analyzed dust absorption features arising in the molecular cloud material and the diffuse interstellar medium along the lines of sight toward Sgr A* and the Quintuplet sources, GCS 3 and GCS 4. It is evident from the depth of the 3.0 μm H2O and the 4.27 μm CO2 ice features that there is more molecular cloud material along the line of sight toward Sgr A* than toward GCS 3 and GCS 4. In fact, Sgr A* has a rich infrared ice spectrum with evidence for the presence of solid CH4, NH3, and possibly HCOOH. Hydrocarbon dust in the diffuse interstellar medium along the line of sight to the GC is characterized by absorption features centered at 3.4, 6.85, and 7.25 μm. Ground-based studies have identified the 3.4 μm feature with aliphatic hydrocarbons, and ISO has given us the first meaningful observations of the corresponding modes at longer wavelengths. The integrated strengths of these three features suggest that hydrogenated amorphous carbon is their carrier. We attribute an absorption feature centered at 3.28 μm in the GCS 3 spectrum to the C–H stretch in aromatic hydrocarbons. This feature is not detected, and its C–C stretch counterpart appears to be weaker, in the Sgr A* spectrum. A key question now is whether or not aromatics are a widespread component of the diffuse interstellar medium, analogous to aliphatic hydrocarbons.