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