5.4. The global evolution of dust-enshrouded galaxies
Fig. 21 summarizes the current state of knowledge of the strong evolution of the comoving luminosity density contributed by luminous far-IR galaxies, whose emission is redshifted into the submm (Blain et al., 1999b, c; Smail et al., 2002). The derivation of these results was discussed briefly above, and is explained in much more detail in these papers. The results of both the luminosity evolution and hierarchical models, which both include strong luminosity evolution of the population of low-redshift IRAS galaxies out to high redshifts, are fully compatible with the redshift information available for submm galaxy samples. This would not be the case if the redshift evolution of the luminosity density was markedly different from the forms shown in Fig. 21. For example, if the luminosity density of submm galaxies were to match rather than exceed the value denoted by the datapoints at z 1 in Fig. 21, and remain at the same high level out to z 10, then the submm-wave spectrum of the cosmic background radiation (Fig. 11) would tend to be too flat, the intensity of the far-IR background radiation would fall short of the observed level, and the predicted redshift distribution of submm galaxies would be biased strongly to the highest redshifts, which at present seems not to be the case (Smail et al., 2002). The determination of complete redshift distributions for existing samples of submm galaxies, and of more redshifts for individual very luminous galaxies drawn from large, future submm-selected galaxy catalogs, will ultimately allow the evolution of distant dusty galaxies to be traced in detail.