The infrared (IR) spectra of objects associated with dust and gas – including evolved stars, reflection nebulae, the interstellar medium (ISM), star-forming regions, and galaxies out to redshifts of z ∼ 3 – are dominated by emission bands at 3.3, 6.2, 7.7, 8.6 and 11.2 μm, the so-called unidentified infrared (UIR) bands. They are generally attributed to the IR fluorescence of Polycyclic Aromatic Hydrocarbon molecules (PAHs) UV pumped by nearby massive stars. Hence, the UIR band strengths are used to determine the star formation rate in galaxies, one of the key indicators for understanding galaxy formation and evolution. To date, PAHs are among the largest and most complex molecules known in space and emit up to 10% of the total power output of star-forming galaxies.
Space-based telescopes such as the Infrared Space Observatory (ISO) and the Spitzer Space Telescope revealed the richness of the PAH spectrum and provided extensive evidence for significant variability in the PAH spectrum from source to source and spatially within sources. In this talk, I will discuss the spectral characteristics of astronomical PAHs, their dependence on the local environments and the implications for the characteristics of the carriers.