Extragalactic source counts in the Spitzer 24 micron band: what do we expect from ISOCAM 15 micron data and models?

The comparison between the new Spitzer data at 24 µm and the previous ISOCAM data at 15 µm is a key tool to understand galaxy properties and evolution in the infrared and to interpret the observed number counts, since the combination of Spitzer with the Infrared Space Observatory cosmological surveys provides for the first time the direct view of the universe in the infrared up to z gsim 2. We present the prediction in the Spitzer 24 µm band of a phenomenological model for galaxy evolution derived from the 15 µm data. Without any "a posteriori" update, the model predictions seem to agree well with the recently published 24 µm extragalactic source counts, suggesting that the peak in the 24 µm counts is dominated by "starburst" galaxies like those detected by ISOCAM at 15 µm but at higher redshifts (1 lesssim z lesssim 2 instead of 0.5 lesssim z lesssim 1.5).