ISOPHOT CIRB project: FIR source counts at 90, 150 and 180 um

Far-infrared maps obtained with ISOPHOT have been searched for point-like sources. The majority of the 55 sources is believed to be extragalactic and in most cases no previously known sources can be associated with them. Based on the far-infrared spectral energy distributions it is likely that dust-enshrouded, distant galaxies form a significant fraction of the sources. We estimate the number densities of extragalactic sources at 90, 150 and 180 um wavelengths and at flux density levels down to 100 mJy. The counts are compared with models of galaxy evolution. The counts exceed the predictions of current models, even those with strong evolution, and no-evolution models are rejected at a high confidence level. Comparison with recent results from COBE mission indicates that at 90 um the detected sources correspond to over 20% of the extragalactic background light. At longer wavelengths the corresponding fraction is 10%.