Far infrared extragalactic background radiation I. Source counts with ISOPHOT

As a part of the ISOPHOT CIRB (Cosmic Infrared Background Radiation) projec t we have searched for point-like sources in eight fields mapped at two or three wavelengths between 90 mu m and 180 mu m. Most of the 55 sources dete cted are suspected to be extragalactic and cannot be associated with previo usly known objects. It is probable, also from the far-infrared (FIR) spectr al energy distributions, that dust-enshrouded, distant galaxies form a sign ificant fraction of the sources. We present a tentative list of new extragalactic FIR-sources and discuss th e uncertainties involved in the process of extracting point sources from th e ISOPHOT maps. Based on the analyzed data we estimate the number density o f extragalactic sources at wavelengths 90 mu m, 150 mu m and 180 mu m and a t flux density levels down to 100 mJy to be 1.10(5) sr(-1), 2.10(5) sr(-1), and 3.10(5) sr(-1), respectively. Strong galaxy evolution models are in best agreement with our results, alth ough the number of detections exceeds most model predictions. No-evolution models can be rejected at a high confidence level. Comparison with COBE results indicates that the detected sources correspond to greater than or similar to 20% of the extragalactic background light at 90 mu m. At longer wavelengths the corresponding fraction is similar to 10 %.