A new method for ISOCAM data reduction - II. Mid-infrared extragalactic source counts in the ELAIS Southern field

We present the 15-mum extragalactic source counts from the Final Analysis Catalogue of the European Large Area ISO Survey (ELAIS) Southern hemisphere field S1, extracted using the Lari method. The large number of extragalactic sources (similar to350) detected over this area between about 0.5 and 100 mJy guarantee a high statistical significance of the source counts in the previously poorly covered flux density range between IRAS and the Deep ISOCAM Surveys. The bright counts in S1 (S-15 mum greater than or similar to 2 mJy) are significantly lower than other published ISOCAM counts in the same flux range and are consistent with a flat, Euclidean slope, suggesting the dominance of a non-evolving population. In contrast, at fainter fluxes (S-15 mum less than or similar to 2 mJy) our counts show a strong departure from no-evolution models, with a very steep super-Euclidean slope down to our flux limit (similar to0.5 mJy). Strong luminosity and density evolutions of the order of, respectively, L proportional to (1 + z)(3.0) and rho proportional to (1 + z)(3.5) are needed at least for the population of star-forming galaxies in order to fit the counts and the redshift distributions observed at different fluxes. A luminosity break around 10(10.8) L. must be introduced in the local luminosity function of starburst galaxies in order to reproduce our sharp increase of the counts below 2 mJy and the redshift distributions observed for 15-mum sources at different flux levels. The contribution of the strongly evolving starburst population (down to 50 muJy) to the 15-mum cosmic background is estimated to be similar to2.2 nW m(-2) sr(-1), which corresponds to similar to67 per cent of the total mid-infrared background estimate.