Cosmic infrared background

Cosmic infrared background is infrared radiation caused by stellar dust. Cosmic infrared background is infrared radiation caused by stellar dust. Recognizing the cosmological importance of the darkness of the night sky (Olbers' paradox) and the first speculations on an extragalactic background light dates back to the first half of the 19th century. Despite its importance, the first attempts were made only in the 1950-60s to derive the value of the visual background due to galaxies, at that time based on the integrated starlight of these stellar systems. In the 1960s the absorption of starlight by dust was already taken into account, but without considering the re-emission of this absorbed energy in the infrared. At that time Jim Peebles pointed out, that in a Big Bang-created Universe there must have been a cosmic infrared background (CIB) – different from the cosmic microwave background – that can account for the formation and evolution of stars and galaxies. In order to produce today's metallicity, early galaxies must have been significantly more powerful than they are today. In the early CIB models the absorption of starlight was neglected, therefore in these models the CIB peaked between 1–10μm wavelengths. These early models have already shown correctly that the CIB was most probably fainter than its foregrounds, and so it was very difficult to observe. Later the discovery and observations of high luminosity infrared galaxies in the vicinity of the Milky Way showed, that the peak of the CIB is most likely at longer wavelengths (around 50μm), and its full power could be ~1−10% of that of the CMB. As Martin Harwit emphasized, the CIB is very important in the understanding of some special astronomical objects, like quasars or ultraluminous infrared galaxies, which are very bright in the infrared. He also pointed out, that the CIB cause a significant attenuation for very high energy electrons, protons and gamma-rays of the cosmic radiation through inverse Compton scattering, photopion and electron-positron pair production. In the early 1980s there were only upper limits available for the CIB. The real observations of the CIB began after the era of astronomical satellites working in the infrared, started by the Infrared Astronomy Satellite (IRAS), and followed by the Cosmic Background Explorer (COBE), the Infrared Space Observatory (ISO) and by the Spitzer Space Telescope. Exploration of the CIB was continued by the Herschel Space Observatory, launched in 2009. The Spitzer wide area surveys have detected anisotropies in the CIB. A summary on the history of CIB research can be found in the review papers by M.G. Hauser and E. Dwek (2001) and A. Kashlinsky (2005). 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 redshift z = 1...2, and was 10 to 50 times the average value today. These galaxies in the z = 1...2 redshift range give 50 to 70 percent of the full brightness of the CIB.