Diffuse interstellar bands.

Diffuse interstellar bands (DIBs) are absorption features seen in the spectra of astronomical objects in our galaxy. They are caused by the absorption of light by the interstellar medium. More than 200 bands are seen, in ultraviolet, visible and infrared wavelengths.

The origin of DIBs was unknown and hotly disputed for many years, and the DIBs were long believed to be due to polycyclic aromatic hydrocarbons and other large carbon-bearing molecules. However, no agreement of the bands could be found with laboratory measurements or with theoretical calculations.

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 extinction. Extinction is caused by dust in the interstellar medium, 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 C¹² atoms and one C¹³ atom, much less will contain one C¹² and two C¹³s, and a very small fraction will contain three C¹³ 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.

In recent years, very high resolution spectrographs on the world's most powerful telescopes have been used to observe and analyse DIBs . Spectral resolutions of 0.005 nm are now routine using instruments at observatories such as the European Southern Observatory at Cerro Paranal, Chile, and the Anglo-Australian Observatory in Australia, and at these high resolutions, many DIBs are found to contain considerable sub-structure.