The mid-infrared spectrum of the Galactic Center - a starburst nucleus

Using the Michelson interferometer on the Midcourse Space Experiment (MSX), we have taken spectra of many positions in the central 25' of the Galactic Center (GC) with a 6' x 9' FOV. The spectral coverage was 380 to 1700 cm(-1) (6 to 26 mu m) and the resolution was similar to 21 cm(-1) The spectra exhibit strong UIR/PAH features at 6.2,7.7, 8.6, and 11.3 mu m, in addition to the ionic lines of [Ne II] at 12.8 mu m, [S III] 18.7 mu m, and [Ar II] 6.98 mu m, There are deep silicate absorption features at 10 and 18 mu m and a cold continuum increasing at the longest wavelengths. Additional weak features are present in the spectra. We discuss the variation in the extinction at 10 mu m as a function of location in the GC. Compared to the MSX spectrum of the Orion Nebula (Simpson et al. 1998), smoothed to the same resolution and multiplied by the estimated GC extinction, the GC spectra have similar PAH features, but the Orion Nebula also has strong lines of [Ne III] 15.6 mu m, [S IV] 10.5 mu m, and [Ar III] 8.99 mu m and its 25 mu m continuum is stronger (colder). Thus, the GC exhibits the mid-TR spectrum of a low excitation H II region and a nearby molecular cloud with a surface photodissociation region (PDR). This is in excellent agreement with the canonical model of a starburst nucleus in which the hot stars and molecular clouds are randomly distributed. The outer surfaces of the clouds are photodissociated and ionized by the photons from the stars located outside the clouds. The PAH molecules are transiently heated by the stellar photons. Since the exciting stars are located well outside the clouds, the radiation field is dilute compared to a newly-formed blister ii II region like Orion; this dilute radiation field causes the relatively low excitation of the ionic lines.