mm observations of IRAS galaxies: dust properties, luminosity functions and contributions to the sub-mm background

We have studied the FIR/{\it mm} spectrum of IR galaxies by combining IRAS photometry with new {\it mm} data on a complete southern IRAS galaxy sample. The observed spectra and a dust model emphasize a dicothomy in the galaxy population: half of the objects with a lot of warm dust are characterized by higher values of the bolometric (UV-FIR) luminosity, of the dust-to-gas mass ratio, of the dust optical depths and extinction, while those dominated by cold ({\it cirrus}) dust show opposite trends. From these data we derive the {\it mm} luminosity function of galaxies and estimate their contribution to the sub-{\it mm} background (BKG)