25 years of PAH hypothesis

Strong IR emission features at 3.3, 6.2, 7.7, 8.6, and 11.2 μm are a common characteristic of the interstellar medium of the Milky Way and nearby galaxies and out to redshifts of3. Here, we review the history of the PAH hypothesis which attributes these emission features to vibrational fluorescence of large (~50 carbon-atom Polycyclic Aromatic Hydrocarbon molecules pumped by ultraviolet photons from nearby stars or the average interstellar radiation field. Over the last 25 years, our insight in the characteristics of these molecules and their role in the Universe has greatly improved and the PAH hypothesis is alive and well; not in the least due a remarkable adaptability. Not surprisingly, the precise characteristics of these species remains to be defined