The PAH hypothesis after 25 years
  The infrared spectra of many galactic and extragalactic objects are dominated
by emission features at 3.3, 6.2, 7.7, 8.6 and 11.2 \mu m. The carriers of
these features remained a mystery for almost a decade, hence the bands were
dubbed the unidentified infrared (UIR) bands. Since the mid-80's, the UIR bands
are generally attributed to the IR fluorescence of Polycyclic Aromatic
Hydrocarbon molecules (PAHs) upon absorption of UV photons -- the PAH
hypothesis. Here we review the progress made over the past 25 years in
understanding the UIR bands and their carriers.
