SPECTROPHOTOMETRY OF UIR BANDS IN THE DIFFUSE EMISSION OF THE GALACTIC DISK

The spectrum of the unidentified infrared (UIR) emission bands between 5.8 and 11.6 mu m has been observed for the first time for the diffus e emission of the galactic disk where the interstellar radiation field (ISRF) is quite low. The UIR bands at 6.2, 7.7, 8.6, and 11.3 mu m ha ve absolute intensities which are similar to 1/10th of the values obse rved in planetary and reflection nebulae. However, the intensity ratio s and band widths are similar, pointing to a common carrier for the UI R bands over a large range of different environmental parameters in th e ISM. This mid-IR emission from a low-ISRF medium and the absence of any detectable continuum emission at 10 mu m in the diffuse galactic s pectra go against any significant contribution from grains at equilibr ium temperature or emission during temperature spikes of very small si licate grains; it supports the identification of the UIR bands as bein g due to the PAH molecules.