Detection of the Mid-Infrared Unidentified Bands in the Diffuse Galactic Emission by IRTS

This letter presents the first results of the mid-infrared spectrum (4.5–11.7 μ m) of the diffuse emission near to the galactic plane (l ∼ 50°; |b| ≤ 5° obtained by the mid-infrared spectrometer on board the Infrared Telescope in Space mission. The results clearly demonstrate the presence of unidentified infrared emission (UIR) bands at 6.2, 7.7, 8.6, and 11.3 μm in the diffuse galactic emission. The distribution of the UIR bands correlates well with the IRAS 100 μm map, suggesting that the UIR-band emitting material is well mixed with the classical dust grains in general interstellar space. The present results indicate that more than 70% of the IRAS 12 μm flux diffuse emission is emitted in the spectral range 8–11.7 μm, and that most of the 12 μm excess comes from the features associated with the UIR bands.