MILLIMETRIC PROPERTIES OF IRAS GALAXIES - III - LUMINOSITY FUNCTIONS,SUB-MM COUNTS AND CONTRIBUTIONS TO THE SKY BACKGROUND

We exploit observations at 1.25 mm with the ESO-SEST telescope of a so uthern galaxy sample, selected from the IRAS PSC and complete to S-60 = 2 Jy, to derive the FIR and mm luminosity functions and the conditio nal probability distributions of FIR and mm luminosity of galaxies. Th e reliability of these estimates is ensured by the good observed corre lation of the far-infrared and mm emissions. This detailed knowledge o f the millimetric properties of galaxies is used to simulate the extra galactic sub-mm sky (background intensity, small-scale anisotropy sign als and discrete source statistics), which is the target of a variety of ground-based and space observatories. We find, in particular, that a recent tentative detection of a sub-mm background would require, if confirmed, strong evolution with cosmic time of the galaxy long-wavele ngth emissivity. We finally discuss ways to test such evolution with p resent and forthcoming facilities: while emphasizing the difficulty of achieving this with large mm telescopes on the ground (because of the poor atmospheric conditions of current sub-mm sites), we mention an i nteresting opportunity with the long-wavelength camera on ISO. Prelimi nary results of deep surveys, both from space and from the ground, see m indeed to require excess emission in the past by dusty galaxies with respect to no-evolution predictions.