A peculiarity of galaxy spectra in the mm and sub-mm region is the very steep rise to shorter wavelengths following the strong spectral dependence of dust emissivity. This is the only spectral region where such a steep spectral shape is observed. The effect of the redshift on the location of the peak implies that it is possible to select high-redshift galaxies at a given flux level.
Figure 1.19 compares the diffuse background for the 2 extreme models of figure 1.18, together with other observational information about the FIR/sub-mm/mm backgrounds. We see that the isotropic sub-mm component found by Puget et al. (1996) in the COBE/FIRAS data (Hauser et al. 1998, Fixsen et al. 1998) is fitted by a PLE model, where the bulk of star-formation activity is confined to z 3. In contrast, galaxy formation is assumed to be a continuous process in the DLE model, with giant galaxies forming at z~ 2. As a result of the decrease in the number density of galaxies with increasing redshift, and of the absence of cold dust in the ISM, the predicted sub-mm background is much smaller.
Recently, the FIR background detected by COBE FIRAS and DIRBE instruments has been confirmed by the ISOPHOT instrument aboard the Infrared Space Observatory (ISO). The so-called FIRBACK deep survey carried out by ISOPHOT found a source density for objects with a flux above 200 mJy that exceeds the counts expected for sources detected in the IRAS deep surveys with a similar flux by about an order of magnitude (see Puget et al. 1999).