We present the first results obtained from the identification of ~30,000 sources in the Spitzer/24?μm observations of the COSMOS field at S 24 μm??80?μJy. Using accurate photometric redshifts (σ z ?~ 0.12 at z?~ 2 for 24?μm sources with i +??25?mag AB) and simple extrapolations of the number counts at faint fluxes, we resolve with unprecedented detail the buildup of the mid-infrared background across cosmic ages. We find that ~50% and ~80% of the 24?μm background intensity originate from galaxies at z??1 and z??2, respectively, supporting the scenario where highly obscured sources at very high redshifts (z??2) contribute only marginally to the cosmic infrared background. Assuming flux-limited selections at optical wavelengths, we also find that the fraction of i +-band sources with 24?μm detection strongly increases up to z?~ 2 as a consequence of the rapid evolution that star-forming galaxies have undergone with look-back time. Nonetheless, this rising trend shows a clear break at z?~ 1.3, probably due to k-correction effects implied by the complexity of spectral energy distributions in the mid-infrared. Finally, we compare our results with the predictions from different models of galaxy formation. We note that semianalytical formalisms currently fail to reproduce the redshift distributions observed at 24?μm. Furthermore, the simulated galaxies at S 24 μm??80?μJy exhibit R–K colors much bluer than observed and the predicted K-band fluxes are systematically underestimated at z??0.5. Unless these discrepancies mainly result from an incorrect treatment of extinction in the models they may reflect an underestimate of the predicted density of high-redshift massive sources with strong ongoing star formation, which would point to more fundamental processes and/or parameters (e.g., initial mass function, critical density to form stars, feedback,...) that are still not fully controlled in the simulations. The most recent backward evolution scenarios reproduce reasonably well the flux/redshift distribution of 24?μm sources up to z?~ 3, although none of them is able to exactly match our results at all redshifts.
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