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 μm, 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.