SOURCE COUNTS AND BACKGROUND RADIATION

Our present understanding of the extragalactic source counts and background radiation at infrared and sub-mm wavelengths is reviewed. Available count data – coming in particular from a very deep survey by ISO in the near- and mid-IR and from deep IRAS surveys at longer wavelengths – are used to constrain evolutionary models of galaxies and Active Nuclei. The extragalactic IR background radiation (CIRB), on the other hand, provides crucial information on the integrated past IR emissivity, including sources so faint to be never accessible. Two spectral bands, the near-IR and sub-millimeter – where local foregrounds are at the minimum – are suited to search for the CIRB. These cosmological windows are ideal to detect redshifted photons from the two most prominent broad emission features in galaxy spectra: the stellar photospheric 1 micron peak and the one at 100 micron due to dust re-radiation. The recently claimed detection of an isotropic diffuse component at λ ≃ 100 − 200 �m would support, whenever confirmed, the evidence for strong cosmic evolution from z = 0 to z ∼ 2 of faint gas-rich late-type galaxies, as inferred from direct long-λ counts. Furthermore, an equally intense CIRB flux measured in the wavelength range 200 to 500 �m may be in support of models envisaging a dust-enshrouded phase during formation of spheroidal galaxies (e.g. Franceschini et al., 1994): the background spectrum at such long wavelengths would indicate an high redshift (z > 2 − 3) for this active phase of star formation, implying a relevant constraint on structure formation scenarios. We argue that, given the difficulties for sky surveys in this spectral domain, only sporadic – and probably inconclusive – tests of these ideas will be achieved in the next several years. Only a mission like FIRST, in combination with large mm arrays and optical telescopes, will allow to thoroughly investigate, via photometric and spectroscopic surveys, these IR-bright early phases in the evolution of galaxies and AGNs.