Galaxy evolution from deep multi-wavelength Infrared surveys: a prelude to Herschel

Abstract: [abridged] At the end of the Spitzer cryogenic mission and the onset of the Herschel era, we review our current knowledge on galaxy evolution at IR wavelengths. We also develop new tools for the analysis of background fluctuations to constrain source counts in regimes of high confusion. We analyse a large variety of new data on galaxy evolution and high-z source populations from Spitzer surveys, as well as complementary data from sub-mm (BLAST) and millimetric ground-based observations. These data confirm earlier indications about a very rapid increase of galaxy volume emissivity up to z~1. This is the fastest evolution rate observed for galaxies at any wavelengths. The observed Spitzer counts at 24 micron require a combination of fast evolution for the dominant population and a bumpy spectrum with substantial PAH emission at z~1 to 2. Confusion-limited number counts at longer wavelengths confirm these results. All the present data require that the fast observed evolution from z=0 to 1 flattens around redshift 1 and then keeps approximately constant up to z~2.5 at least. The present-day IR data provide clear evidence for the existence of a distinct population of very luminous galaxies becoming dominant at z > 1. Their cosmological evolution, peaking around z~2, shows a faster decay with cosmic time than lower luminosity systems, whose maximal activity is set around z~1, then supporting an earlier phase of formation for the most luminous and massive galaxies. From a comparison of our results on the comoving IR emissivity with recent estimates of the redshift-dependent stellar mass functions of galaxies, we infer that a large fraction (>=50%) of the IR activity at z > 1 should be due to obscured AGN accretion and that concomitant SF in high-z luminous sources should follow a top-heavy IMF.