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

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 ecl iptic polar region [ApJS 63 (1987) 311]. Exploiting the higher angular reso lution of ISOCAM to deal with confusion effects and properly correcting the fluxes for observational biases, we show that 60 mum counts can be reliabl y determined only above 80 mJy. We have also derived the distributions of 1 5 to 60 mum and of 100 to 60 mum flux density ratios for the sample and use d them to extrapolate the 60 mum counts of galaxies to the ISOCAM LW3 15 mm and to the ISOPHOT C100 90 mm bands. Our results cover a flux density rang e 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 be ing 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. (C) 2001 Published by Elsevier Science B. V.