The PAH hypothesis after 25 years

TL;DR

This paper reviews 25 years of research on the PAH hypothesis, which explains infrared emission features in space as caused by polycyclic aromatic hydrocarbons absorbing UV light and fluorescing.

Contribution

It summarizes the progress and evidence supporting the PAH hypothesis for the origin of unidentified infrared bands over the past quarter-century.

Findings

Strong correlation between PAH presence and IR emission features

Advances in spectral analysis supporting PAH carriers

Progress in understanding PAH excitation and emission mechanisms

Abstract

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.