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 m has been observed for the first time for the diffuse 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 m have absolute intensities which are 1/10th of the values observed in planetary and reflection nebulae. However, the intensity ratios and band widths are similar, pointing to a common carrier for the UIR bands over a large range of different environmental parameters in the ISM. This mid-IR emission from a low-ISRF medium and the absence of any detectable continuum emission at 10 m in the diffuse galactic spectra go against any significant contribution from grains at equilibrium temperature or emission during temperature spikes of very small silicate grains; it supports the identification of the UIR bands as being due to the PAH molecules.