Origin of The Cosmic Infrared Background
One of the most important questions about the CIB is the source of its energy. In the early models the CIB was built up from the redshifted spectra of the galaxies found in our cosmic neighborhood. However, these simple models could not reproduce the observed features of the CIB. In the baryonic material of the Universe there are two sources of large amounts of energy: nuclear fusion and gravitation.
Nuclear fusion takes place inside the stars, and we can really see this light redshifted: this is the main source of the cosmic ultraviolet- and visual background. However, a significant amount of this starlight is not observed directly. Dust in the host galaxies can absorb it and re-emit it in the infrared, contributing to the CIB. Although most of today's galaxies contain little dust (e.g. elliptical galaxies are practically dustless), there are some special stellar systems even in our vicinity which are extremely bright in the infrared and at the same time faint (often almost invisible) in the optical. These ultraluminous infrared galaxies (ULIRGs) are just in a very active star formation period: they are just in a collision or in a merge with another galaxy. In the optical this is hidden by the huge amount of dust, and the galaxy is bright in the infrared due to the same reason. Galaxy collisions and mergers were more frequent in the cosmic past: the global star formation rate of the Universe peaked around z=1..2, and was 10...50 times the average value today. These galaxies in the z=1...2 redshift range gives the 50...70% of the full brightness of the CIB.
Another important component of the CIB is the infrared emission by quasars. In these systems most of the gravitational potential energy of the matter falling into the central black hole is converted into X-rays, which would escape unless it is absorbed by the dust torus of the accretion disc. This absorbed light is again re-emitted in the infrared, and in total gives about 20-30% of the full power of the CIB; however at some specific wavelengths this is the dominant source of CIB energy.
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