PAH Evolution in the Harsh Environment of the ISM

We review observational results of PAH emission in harsh interstellar environments, which are mostly based on recent works by Spitzer and AKARI. The harsh environments include shock regions in our Galaxy, the ionized superwinds and haloes of external galaxies, and the hot plasmas of elliptical galaxies. Owing to the unprecedented high sensitivity for PAH emission with Spitzer and AKARI, it is found that an appreciable amount of PAHs are present in some cases with such hostile conditions. Some of them exhibit unusual PAH interband strength ratios, reflecting either evolution of PAHs or physical conditions of the ISM. The distribution of the PAH emission, as compared to that of dust emission, is shown to discuss their ways of evolution and survival.