IRTS Observation of the Unidentified 3.3-Micron Band in the Diffuse Galactic Emission

We discuss the observation of unidentified infrared (UIR) band emission at a wavelength of 3.3 μm from the galactic plane region (42° < l < 55°, |b| < 5° with the Near-Infrared Spectrometer (NIRS) on board the Infrared Telescope in Space (IRTS). The result shows that the 3.3 μm UIR band emission in the diffuse interstellar medium has been clearly detected, and its intensity correlates well with the IRAS 12 μm and 100 μm intensities everywhere in the observed region. The ratios of the 3.3 μm UIR band intensity (ΔλIλ) to the 12 μm and 100 μm intensities (λIλ) are (8.2 ± 2.5) × 10−3 and (2.9 ± 0.9) 10−3, respectively. The good correlation with the 100 μm intensity indicates that the 3.3 μm UIR band emitter is well mixed with the classical dust grains with sub-micron size in the general interstellar medium.

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