Extragalactic Source Counts in the Spitzer 24 Micron Band: What Do We Expect from ISOCAM 15 Micron Data and Models?

Abstract

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>~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<~z<~2 instead of 0.5<~z<~1.5).