Spectroscopic follow-up of FIRBACK-South bright galaxies

We have performed optical spectroscopy of the brightest 170 mum sources in the FIRBACK South Marano (FSM) field. The spectroscopic sample is 90% complete at the 4sigma level. Analysis of spectra and comparison with spectra of a faint IRAS (60 mum selected) reference sample is conducted simultaneously. The sources in both samples are characterized by a predominance of emission-line spectra and moderate IR luminosities (10(10.5) < L-IR < 10(12)L(.)) The fraction of AGN's is low (about 15%) and the majority of sources are nearby (z < 0.3), dusty, star forming galaxies, with moderate star formation rates (a few 10 M-. per year). The infrared emission of the FSM galaxies shows a colder spectral energy distribution than was expected. The galaxies in both samples ( IRAS faint sources and FSM) are essentially undistinguishable with the available data, and seem to represent a population of closeby, cold, star forming galaxies rather neglected up to now. Although their contribution to the far-IR background seems to be low, they deserve a more detailed study on their own to assess their importance in the chemical evolution of the local universe. The contribution of fainter, presumably more distant, galaxies to the far-IR background will be discussed when we will have completed the follow-up of the fainter part of this 170 μm survey.