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 90μm and 180μm. Most of the 55 sources detected are suspected to be extragalactic and cannot be associated with previously known objects. It is probable, also from the far-infrared (FIR) spectral energy distributions, that dust-enshrouded, distant galaxies form a significant fraction of the sources. We present a tentative list of new extragalactic FIR-sources and discuss the uncertainties involved in the process of extracting point sources from the ISOPHOT maps. Based on the analyzed data we estimate the number density of extragalactic sources at wavelengths 90μm, 150μm and 180μm and at flux density levels down to 100 mJy to be 1·105 sr−1, 2·105 sr−1, and 3·105 sr−1, 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 the detected sources correspond to >∼20% of the extragalactic background light at 90μm. At longer wavelengths the corresponding fraction is ∼10%.