a1 Department of Physics and Astronomy, University of Western Ontario, London, ON N6A 3K7, Canada email: firstname.lastname@example.org
a2 SETI Institute, 189 Bernardo Ave., Suite 100, Mountain View, CA 94043, USA email: email@example.com
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 μm. 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.