ISOCAM observations of the deep IRAS 60 micron sample in the NEP region*1II. Comparison of ISO and IRAS galaxy counts

Abstract We have determined IRAS flux densities at positions of sources detected by our ISOCAM LW3 observations of the IRAS deep survey sample in the north ecliptic polar region [ApJS 63 (1987) 311]. Exploiting the higher angular resolution of ISOCAM to deal with confusion effects and properly correcting the fluxes for observational biases, we show that 60 μm counts can be reliably determined only above 80 mJy. We have also derived the distributions of 15 to 60 μm and of 100 to 60 μm flux density ratios for the sample and used them to extrapolate the 60 μm counts of galaxies to the ISOCAM LW3 15 μm and to the ISOPHOT C100 90 μm bands. Our results cover a flux density range intermediate between those of the European Large Area ISO Survey and the IRAS Faint Source Survey, hence allowing to establish a direct link between ISO and IRAS surveys. The present estimate has the further advantage of being automatically rid of the stellar contamination to the galaxy counts and provides a crucial boundary to constrain evolutionary properties of faint mid- and far-IR sources.