Polycyclic Aromatic Hydrocarbons (PAHs) are the most likely carriers proposed to account for the Aromatic Infrared Bands observed in emission between 3.3 and 12.7 μm, fulfilling both the chemical (presence of CC and CH aromatic bonds) and excitation constraints. We explain here how the emission spectrum of such species in astronomical environments can be modelled, to obtain for each infrared band both its intensity and its detailed profile.
The PAH model suffers from the lack of identification of individual species. Two strategies are described here that are currently being used to progress in this identification process. The first idea is that features in the far IR are much more specific to the exact molecular identity. Therefore their search with the coming Herschel Space Observatory will provide an unambiguous way to identify these molecules through their IR emission. Second, a reflection on where interstellar PAHs come from and how they evolve due to environmental conditions (UV irradiation, gas and dust interactions) is also necessary to get more insights into the nature of interstellar PAH candidates. In particular, recent studies have emphasized a chemical link between PAHs and very small grains in photodissociation regions.
Finally, we illustrate in this paper how progress in this field can only be attained by a synergy between astronomical observations, fundamental studies, both theoretical and experimental, and models.