Organic dust in galaxies

Takashi Onaka

University of Tokyo, Japan

A family of distinct infrared emission bands observed in the near- to mid-infrared spectra of celestial objects is generally attributed to polycyclic aromatic hydrocarbons (PAHs) or small particles containing PAH structure. The major bands are seen at 3.3, 6.2, 7.7, 8.6, and 11.2mm together with several minor features. They are observed not only in Galactic objects, but also appear conspicuously in mid-infrared spectra of star-forming galaxies, suggesting that they are important members of organic matter in the Universe. Since they are thought to be the smallest member of interstellar dust and thus very sensitive to the environmental conditions, their variations will provide useful information for the study of processing in the interstellar medium. Recent advent of infrared satellites, Spitzer and AKARI, has allowed significant progress in our understanding of their processing in the interstellar medium; however, the exact nature of the carriers is still under debate and there are a number of enigmas remaining particularly in their formation and destruction processes. In this review, the latest results on the organic band emission in our Galaxy and galaxies obtained with the infrared satellites are reviewed together with other carbon-bearing solid species in space.

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