A Tale of Two Mysteries in Interstellar Astrophysics: The 2175 Angstrom Extinction Bump and Diffuse Interstellar Bands

TL;DR

This study investigates the potential link between the 2175 Angstrom extinction bump and diffuse interstellar bands (DIBs), finding no correlation and suggesting different PAH populations may be responsible for each feature.

Contribution

The paper provides a comprehensive literature analysis showing no correlation between the 2175 Angstrom bump and DIBs, challenging the hypothesis that PAHs are the common carrier for both.

Findings

No correlation between 2175 Angstrom feature and DIBs strengths.

Different PAH populations may produce the two features.

Supports the idea of distinct carriers for the bump and DIBs.

Abstract

The diffuse interstellar bands (DIBs) are ubiquitous absorption spectral features arising from the tenuous material in the space between stars -- the interstellar medium (ISM). Since their first detection nearly nine decades ago, over 400 DIBs have been observed in the visible and near-infrared wavelength range in both the Milky Way and external galaxies, both nearby and distant. However, the identity of the species responsible for these bands remains as one of the most enigmatic mysteries in astrophysics. An equally mysterious interstellar spectral signature is the 2175 Angstrom extinction bump, the strongest absorption feature observed in the ISM. Its carrier also remains unclear since its first detection 46 years ago. Polycyclic aromatic hydrocarbon (PAH) molecules have long been proposed as a candidate for DIBs as their electronic transitions occur in the wavelength range where DIBs are often found. In recent years, the 2175 Angstrom extinction bump is also often attributed to the \pi--\pi* transition in PAHs. If PAHs are indeed responsible for both the 2175 Angstrom extinction feature and DIBs, their strengths may correlate. We perform an extensive literature search for lines of sight for which both the 2175 Angstrom extinction feature and DIBs have been measured. Unfortunately, we found no correlation between the strength of the 2175 Angstrom feature and the equivalent widths of the strongest DIBs. A possible explanation might be that DIBs are produced by small free gas-phase PAH molecules and ions, while the 2175 Angstrom bump is mainly from large PAHs or PAH clusters in condensed phase so that there is no tight correlation between DIBs and the 2175 Angstrom bump.