2002MNRAS.335..831G

We present the 15-µm 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 (∼350) 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 µm_≳ 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 µm_≲ 2 mJy) our counts show a strong departure from no-evolution models, with a very steep super-Euclidean slope down to our flux limit (∼0.5 mJy). Strong luminosity and density evolutions of the order of, respectively, L ∝ (1 + z ) ^3.0 and ρ∝ (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-µm sources at different flux levels. The contribution of the strongly evolving starburst population (down to 50 µJy) to the 15-µm cosmic background is estimated to be ∼2.2 nW m ^–2 sr ^–1, which corresponds to ∼67 per cent of the total mid-infrared background estimate.