Optical searches are biased towards unobscured, active and/or starbursting galaxies (including low-mass galaxies in an active phase). Near-infrared surveys, in turn, detect primarily photospheric emission of late-type stars (red giants) and hence are biased towards early-type (E/S0/Sa) galaxies. Dust, which is found in most galaxies, includes a substantial fraction of all metals and strongly affects the observability of galaxies at different wavelengths. In this context, a major breakthrough in our understanding of galaxies has been made by the IRAS survey. Nevertheless, because the longest wavelength observed was m, IRAS detected primarily the warm dust component in low redshift galaxies, heated by the intense radiation fields from star-forming regions. IRAS was not sensitive to either cold dust or high-redshift galaxies.
Only future extensive sub-millimetre surveys, such as the one planned in the PLANCK mission, can determine how much of the stellar radiation in galaxies is either reprocessed by cold dust or redshifted to submillimetre wavelengths. PLANCK is expected to provide broad-band spectral information for several thousands of optical- and IRAS-selected galaxies (see Table 1.1), and also to test for the existence of galaxies optically obscured by cold dust. The observations will reveal the spectral behavior in this poorly-known frequency range. In addition to the study of energy balance in normal and starbursting galaxies, the multi-frequency sample will set constraints on dust properties (temperature, emissivity).
Statistical information on the various classes of objects will allow the definition of templates for modeling galaxy formation and evolution. Furthermore, a large sample of local sub-mm galaxies is required for precise and unbiased estimates of the local luminosity functions, a crucial step when interpreting the evolution of galaxies at infrared wavelengths. Studies of the angular correlation function will complement the analyses made from optical or IRAS surveys, and could help to test the evolution of galaxy clustering with cosmic time over a fraction of the Hubble time corresponding to .
Figure 1.18 summarizes some specific results about the predicted galaxy counts and redshift distributions relevant to the sub-mm observations of PLANCK, based on IRAS luminosity functions and theoretical FIR/submm/mm spectra designed to fit IRAS colors. There is a large uncertainty on dust emission at sub-mm wavelengths. In order to bracket the uncertainties, two extreme evolutionary models have been assumed based on pure luminosity evolution (PLE) and density plus luminosity evolution (DLE). The DLE model includes only the warm dust seen by IRAS while the PLE model also includes a cold dust component consistent with the 1.25 mm observations of Franceschini & Andreani (1995). The counts and redshift distributions are estimated with reference to the m channel which, given the combination of sensitivity and angular resolution, gives the largest sample of infrared galaxies. The large difference in the predictions of these two models at m, given that they agree with the IRAS counts at m, illustrates how crucial submillimetre observations are in constraining models of galaxy evolution. These models show that a significant fraction of galaxies detected by PLANCK are expected to lie at z>0.2, in marked contrast to the low median redshift () of galaxies detected by IRAS. Note that Figure 1.18 shows that the behavior of the counts depart from an Euclidean law even for weakly-evolving galaxies (e.g. the normal spirals).
The conclusion, although model dependent to some extent, is that PLANCK surveys would give access to an intermediate redshift range that is complementary to other existing all-sky catalogues.