Raman Scattering as the Key Link Between Unidentified Infrared Bands, Diffuse Interstellar Bands, and Extended Red Emission

TL;DR

This paper proposes that Raman scattering by atomic hydrogen explains the connection between unidentified infrared bands, diffuse interstellar bands, and extended red emission in interstellar clouds, unifying these phenomena.

Contribution

It introduces the novel idea that Raman scattering by atomic hydrogen accounts for multiple interstellar spectral features, linking them as different aspects of the same process.

Findings

Raman scattering by hydrogen is detected in various astronomical objects.

It can explain the properties of DIBs, ERE, and UIBs.

Observation conditions depend on radiation field, dust extinction, and geometry.

Abstract

This paper calls attention to the relevance of Raman scattering by atomic hydrogen to three optical and near/mid-infrared spectral features of HI clouds: extended red emission (ERE), diffuse interstellar bands (DIBs), and the unidentified infrared bands (UIBs). DIBs, ERE, and UIBs are observed predominantly at the edge of HI clouds, are manifestly related, and remain poorly understood. Their salient properties correspond to two major characteristics of HI Raman scattering: unusual line broadenings and a concentration of the Raman scattered ultraviolet continuum in the vicinity of hydrogen's optical and infrared transitions. Raman scattering by atomic hydrogen has now been detected in several object classes where the spectral features are observed, and I argue that it can account for all three features. I further identify three factors that condition observation of Raman scattering in HI clouds, and thus of DIBs, ERE, and UIBs: the hardness of the radiation field, interstellar dust extinction, and the geometry of the observation. The geometry determines whether complete forward scattering, yielding DIBs, or scattering at large angles, yielding ERE in the vicinity of H{\alpha} and UIBs in the infrared spectrum, will be observed. ERE results from Raman scattering of photons near Ly\b{eta} and UIBs from excitation of hydrogen atoms close to the ionization limit. DIBs, ERE, UIBs are thus different facets of the same interstellar phenomenon: Raman scattering by atomic hydrogen.