We present the 15 \mu m extragalactic source counts from the Final Analysis Catalogue of the European Large Area ISO Survey southern hemisphere field S1 , extracted using the Lari method .
The large number of extragalactic sources ( \sim 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 ~ { } \mu m } { { } _ { > } \atop { } ^ { \sim } } 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 ~ { } \mu m } { { } _ { < } \atop { } ^ { \sim } } 2 mJy ) our counts show a strong departure from no evolution models , with a very steep super-Euclidean slope down to our flux limit ( \sim 0.5 mJy ) .
Strong luminosity and density evolution of the order of , respectively , L \propto ( 1 + z ) ^ { 3.0 } and \rho \propto ( 1 + z ) ^ { 3.5 } is 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 _ { \odot } 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 \mu m sources at different flux levels .
The contribution of the strongly evolving starburst population ( down to 50 \mu Jy ) to the 15 \mu m cosmic background is estimated to be \sim 2.2 nW m ^ { -2 } sr ^ { -1 } , which corresponds to \sim 67 % of the total mid-infrared background estimate .