the date of receipt and acceptance should be inserted later Abstract. We have performed optical spectroscopy of the brightest 170µm sources in the FIRBACK South Marano (FSM) field. The spectroscopic sample is 90 % complete at the 4σ level. Analysis of spectra and comparison with spectra of a faint IRAS (60 µm 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 < LIR < 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 170µm survey
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