The Nature of The Carrier
The great problem with DIBs, apparent from the earliest observations, was that their central wavelengths did not correspond with any known spectral lines of any ion or molecule, and so the material which was responsible for the absorption could not be identified. A large number of theories were advanced as the number of known DIBs grew, and determining the nature of the absorbing material (the 'carrier') became a crucial problem in astrophysics.
One important observational result is that the strengths of most DIBs are not correlated with each other. This means that there must be many carriers, rather than one carrier responsible for all DIBs. Also significant is that the strength of DIBs is broadly correlated with the interstellar extinction. Extinction is caused by interstellar dust, and so DIBs are likely to be also due to dust or something related to it.
The existence of sub-structure in DIBs supports the idea that they are caused by molecules. Substructure results from band heads in the rotational band contour and from isotope substitution. In a molecule containing, say, three carbon atoms, some of the carbon will be in the form of the carbon-13 isotope, so that while most molecules will contain three carbon-12 atoms, some will contain two 12C atoms and one 13C atom, much less will contain one 12C and two 13Cs, and a very small fraction will contain three 13C molecules. Each of these forms of the molecule will create an absorption line at a slightly different rest wavelength.
The most likely candidate molecules for producing DIBs are thought to be large carbon-bearing molecules, which are common in the interstellar medium. Polycyclic aromatic hydrocarbons, long carbon-chain molecules, and fullerenes are all potentially important.
Read more about this topic: Diffuse Interstellar Band
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