QAUNTITATIVE STUDY OF MID-INFRARED SPECTRA AND MID-INFRARED HYDROCARBON BANDS OF EXTERNAL GALAXIES

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We have analyzed the data taken by IRTS and ISO for the Galactic plane and found that the mid-infrared organic bands do not change their band ratios with the strength of incident radiation field. This result indicates that the interstellar organics are fully ionized contrary to the model predictions. We also found a broad band feature around 22 micron in the Carina data. A similar band is also detected in a supernova remnant and in an external galaxy. We suggest that (1) the 22 micron band carriers may be formed in supernova explosion, (2) it can be used an index for recent star formation activities, and (3) it could affect wide-band photometry. Based on the far-infrared spectroscopy of external galaxies, we found that the gas density increases with the activity and that the far-infrared characteristics do not change with the galaxy morphology. Analysis of infrared camera data indicates that (1) the total 7 micron to 15 micron mid-infrared color is determined by the color of the nucleus for most galaxies, (2) the color decreases for barred galaxies, but (3) there are some galaxies which show low ratios in the extended region, suggesting that the mid-infrared color indicates short-lived activities in galaxies. Investigations of the spectral energy distribution (SED) from mid-infrared to far-infrared further suggest that the SED of the Galactic plane and external galaxies can be accounted for consistently if we use an empirical SED of the Galactic plane. We have established a model that accounts for the mid-infrared characteristics, including the mid-infrared bands, and have achieved the primary goal of this project.