Diffuse interstellar bands (DIBs) are absorption features seen in the spectra of astronomical objects in the Milky Way and other galaxies. They are caused by the absorption of light by the interstellar medium. Circa 500 bands have now been seen, in ultraviolet, visible and infrared wavelengths.
The origin of most DIBs remains unknown, with common suggestions being polycyclic aromatic hydrocarbons and other large carbon-bearing molecules. Only one DIB carrier has been identified: ionised buckminsterfullerene (C60+), which is responsible for several DIBs in the near-infrared. The carriers of most DIBs remain unidentified.
Discovery and history
Much astronomical work relies on the study of spectra - the light from astronomical objects dispersed using a prism or, more usually, a diffraction grating. A typical stellar spectrum will consist of a continuum, containing absorption lines, each of which is attributed to a particular atomic energy level transition in the atmosphere of the star.
The appearances of all astronomical objects are affected by extinction, the absorption and scattering of photons by the interstellar medium. Relevant to DIBs is interstellar absorption, which predominantly affects the whole spectrum in a continuous way, rather than causing absorption lines. In 1922, though, astronomer Mary Lea Heger first observed a number of line-like absorption features which seemed to be interstellar in origin.
Their interstellar nature was shown by the fact that the strength of the observed absorption was roughly proportional to the extinction, and that in objects with widely differing radial velocities the absorption bands were not affected by Doppler shifting, implying that the absorption was not occurring in or around the object concerned. The name diffuse interstellar band, or DIB for short, was coined to reflect the fact that the absorption features are much broader than the normal absorption lines seen in stellar spectra.
The first DIBs observed were those at wavelengths 578.0 and 579.7 nanometers (visible light corresponds to a wavelength range of 400 - 700 nanometers). Other strong DIBs are seen at 628.4, 661.4 and 443.0 nm. The 443.0 nm DIB is particularly broad at about 1.2 nm across - typical intrinsic stellar absorption features are 0.1 nm or less across.
Later spectroscopic studies at higher spectral resolution and sensitivity revealed more and more DIBs; a catalogue of them in 1975 contained 25 known DIBs, and a decade later the number known had more than doubled. The first detection-limited survey was published by Peter Jenniskens and Xavier Desert in 1994 (see Figure above), which led to the first conference on The Diffuse Interstellar Bands at the University of Colorado in Boulder on May 16–19, 1994. Today circa 500 have been detected.
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.
The nature of the carriers
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 strongly 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; however, DIBs, are not likely to be caused by dust grains.