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
%.