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

We have studied the FIR/mm spectrum of IR galaxies by combining IRAS photometry with new 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 (cirrus) dust show opposite trends. From these data we derive the mm luminosity function of galaxies and estimate their contribution to the sub-mm background (BKG).