Description
The FIRBACK (Far Infrared BACKground) survey is one of the deepest imaging surveys carried out at 170{mu}m with ISOPHOT onboard ISO, and is aimed at the study of the structure of the Cosmic Far Infrared Background. This paper provides the analysis of resolved sources. After a validated process of data reduction and calibration, we perform intensive simulations to optimize the source extraction, measure the confusion noise ({sigma}_c_=45mJy), and give the photometric and astrometric accuracies. 196 galaxies with flux S>3{sigma}_c_ are detected in the area of 3.89 square degrees. Counts of sources with flux S>4{sigma}_c_ present a steep slope of 3.3+/-0.6 on a differential ``logN-logS'' plot between 180 and 500mJy. As a consequence, the confusion level is high and will impact dramatically on future IR deep surveys. This strong evolution, compared with a slope of 2.5 from Euclidian geometry, is in line with models implying a strongly evolving Luminous Infrared Galaxy population. The resolved sources account for less than 10% of the Cosmic Infrared Background at 170{mu}m, which is expected to be resolved into sources in the 1 to 10mJy range.
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