Far infrared extragalactic background radiation: I. Source counts with ISOPHOT

As a part of the ISOPHOT CIRB (Cosmic Infrared Background Radiation) project we have searched for point-like sources in eight fields mapped at two or three wavelengths between 90 and 180um. Most sources are previously unknown and based on the far-infrared (FIR) spectral energy distributions dust-enshrouded, distant galaxies form a significant fraction of the sources. We present a list of the new extragalactic FIR-sources and discuss the uncertainties of the source detection from the ISOPHOT maps. We estimate the source number densities at 90, 150 and 180um and at flux density levels down to 100mJy to be 10^5, 2 10^5, and 3 10^5 1/sr, respectively. Strong galaxy evolution models are in best agreement with our results, although 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 at 90um the detected sources correspond to some 20% of the extragalactic background light. At longer wavelengths the corresponding fraction is closer to 10%.