Infrared background in general

The night sky is nowhere empty. There is always some source of light even between the faintest stars or galaxies whose source cannot be easily determined. Astronomers usually refers to this phenomenon as the 'sky background': a (usually) faint glow between interesting objects. The definition of the background always depends on the actual measurement configuration, but there are two main classes. A background can be intrinsically diffuse (like e.g. the gas clouds of our Galaxy) or made of the accumulated light of unresolved sources (like the diffuse glow of the Milky Way for human's eye). In the latter case a better resolution decrease the number of sources in the background. A 'background' limits the detectability of faint sources in two ways. First, sources fainter than the background cannot be observed (photometric criterium) and second, the sources must be far enough from each other to see them separately (source density criterium). These limitations are commonly referred as the 'confusion noise'.

Figure 1: Detectable sources must be brigther than the background level and must be well separated from each other.

Figure 2: Components of the sky background from UV (~0.1µm) to milimeter wavelengths, without the contribution of the extragalactic background (Leinert, 1998)

The background has two main characteristics at a specific wavelength: its absoulte surface brightness and its spatial structure, i.e. the properties of its fluctuations -- the typical shape, depth and frequency of occurence of its 'hills' and 'valleys'. The sky background consits of many interesting components, and its relative weight depends on the wavelength.

In the far infrared the most important astrophysical components of the sky background are: Thermal emission from interplanetary dust (Zodiacal Light) Emission from irregurally shaped interstellar clouds in the Milky Way (galactic cirrus emission) Dust emission from the intergalactic dust in the Local Group The extragalactic background (accumulated light of unresolved galaxies) At long wavelenghts there is a contribution from the cosmic microwave background (the afterglow of the Big Bang), which has to be distinguished from the others.

Figure 3: Components of the far-infrared sky background and their distance scales.

The separation of these components are very challenging and their properties must be well know in order to separate them correctly. Most background studies aim to detect and investigate the properties of the far-infrared extragalatic background (or cosmic far-infrared background, CFIRB), since this component has 'cosmological' importance: it contains information on the formation and evolution of galaxies in the young Universe. However, the study of the other components are also very important. E.g. the investigation of the Zodical Light emission provides information on the evolution of dust in the Solar System and the understanding of the structure of cirrus clouds helps to answer the question about the first steps of star formation.