Astronomical Observations of the PAH Emission Bands

The infrared (IR) spectra of many galactic and extragalactic objects are dominated by emission features at 3.3, 6.2, 7.7, 8.6 and 11.2 μ m, generally attributed to the IR fluorescence of Polycyclic Aromatic Hydrocarbon molecules (PAHs). These PAH bands have been found in a wide variety of environments throughout theUniverse and contain up to 10% of the total power output of star-forming galaxies. Ground-based telescopes, the Infrared Space Observatory (ISO) and the Spitzer Space Telescope revealed a plethora of weaker PAH bands and provided extensive evidence for significant variability in the PAH spectrum from source to source and spatially within sources. Here we review the spectral characteristics of astronomical PAHs, their dependence on the local physical conditions and the implications for the physical and chemical characteristics of the carriers.