Extended Red Emission: Photoluminescence by Interstellar Nanoparticles

Interstellar dust in nebulae and in the diffuse interstellar medium of galaxies contains a component which responds to illumination by ultraviolet photons with efficient luminescence in the 500 nm to 1000 nm spectral range, known as Extended Red Emission (ERE). We review the techniques of detection and the observational characteristics of the ERE in a wide range of astrophysical environments. We then discuss results of the analysis of ERE observations, leading to a set of specific constraints that any proposal for the ERE carrier must confront. Finally, we review specific models that have been advanced over the past two decades to explain the ERE phenomenon. Despite promising progress on several fronts, no completely satisfactory model for the ERE carrier/process exists at this time.