2023ApJ...952..116Z

Photodissociation regions (PDRs) illuminated by high temperature (T > 10⁴ K) radiation fields display characteristic optical to midinfrared spectral features: emission bands that cluster around the blue limits of hydrogen series, an extended red emission (ERE) in the vicinity of Hα, and a continuum in the infrared part of the spectrum only. The spectral footprint of ERE and its intensity at maximum unequivocally designate Raman scattering by hydrogen of photons near Lyβ as responsible for the feature, thus suggesting PDR spectra be considered Raman spectra. This finding challenges the currently accepted hypothesis that yet-to-be-identified complex carbonaceous molecules, such as polycyclic aromatic hydrocarbons, are the source of PDRs' unidentified infrared bands. It has, therefore, significant implications for the field of astrochemistry.