Organic compounds in galaxies

Abstract

The unidentified infrared (UIR) emission bands in the near- to mid-infrared are thought to originate from organic compounds in the interstellar medium. Recent space observations with Spitzer and AKARI have clearly revealed that the UIR bands are commonly seen in external galaxies, including elliptical galaxies, except for very metal-poor dwarf galaxies. They are also detected in extended structures of galaxies, such as extra-planar components and filaments produced by outflows, suggesting that the band carriers are ubiquitous organic compounds in galaxies. Since the UIR bands are prominent features in the infrared spectrum of galaxies and are linked to the star-formation activity, it is highly important to understand the nature, formation, processing, and destruction of the UIR band carriers in galaxies. While there is no systematic variation detected in the UIR spectrum in normal galaxies, significantly low values are derived for the ratio of the 7.7 μm to 11.2 μm bands in elliptical galaxies as well as in galaxies with low-luminosity AGNs compared to normal star-forming galaxies. Relatively low band ratios are also seen in the UIR band spectrum of extended structures in galaxies. If the same mechanism leads to the low band ratio, it would provide important information on the band carrier properties. It should also be noted that the band carriers are believed to be destroyed in a short time scale in environments where low band ratios are detected. The survival and supply processes in these environments are a key to understand the nature of the band carriers. © 2008 International Astronomical Union.