Cosmic infrared background

Cosmic infrared background is infrared radiation caused by stellar dust.

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.

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 they are 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.

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.

The most important foreground components of the CIB are the following:

  • Zodiacal emission: the thermal emission of microscopic dust particles in the Solar System (from near- to mid-infrared)
  • Thermal emission of small asteroids in the Solar System (from near- to mid-infrared)
  • Galactic cirrus emission (far-infrared)
  • Faint galactic stars (in the near-infrared, λ<20μm)
  • Infrared emission of intracluster dust in the Local Group
  • The cosmic microwave background – although physically it is not a “foreground” – is also considered as an important contaminating source of emission at very long infrared wavelengths (λ>300μm)

These components must be separated for a clear CIB detection.

The detection of the CIB is both observationally and astrophysically very challenging. It has a very few characteristics which can be used to separate it from the foregrounds. One major point is, that the CIB must be isotropic, i.e. one has to measure the same CIB value all over the sky. It also lacks suspicious spectral features, since the final shape of its spectrum is the sum of the spectra of sources in the line of sight at various redshifts.

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