The unidentified infrared bands in the diffuse interstellar medium across the galaxy based on the infrared telescope in space mid-infrared spectrometer observation

We present the results of observations of the unidentified infrared (UIR) bands in the diffuse Galactic emission across the Galaxy by the Mid-Infrared Spectrometer (MIRS) on board the Infrared Telescope in Space (IRTS). While previous studies on the UIR bands in the Milky Way were limited to the inner Galactic plane, we extend the observing area to the outer Galactic plane. In this paper we analyze the data of four areas of 8°×8° around the Galactic plane (|b| ≤ 4°; -12° ≤ l ≤ -4°, 44° ≤ l ≤ 52°, -136° ≤ l ≤ -128°, and 168° ≤ l ≤ 176°) and investigate the UIR band intensity relative to the far-infrared (FIR) intensity, as well as the variation in the band profile. Together with the good correlation between the UIR band and the FIR intensities in the four regions, we have found a systematic variation in the UIR-to-FIR ratio such that the ratio becomes larger in the outer Galactic plane than in the inner Galactic plane. In addition, the 8.6 and 11.3 μm UIR bands were found to be stronger relative to the 6.2 and 7.7 μm bands in the outer Galactic plane, which may be related to differences in the structure or physical conditions of the band carriers. We have also found small shifts (̃0.1 μm) in the peak wavelength of each UIR band to shorter wavelengths from the inner Galactic plane to the outer Galactic plane. Possible interpretations of these variations are discussed.