Linking stellar mass and star formation in Spitzer MIPS 24 µm galaxies

International audienceWe present deep Ks 83 µJy (the ~ 80% completeness limit of the Spitzer /GTO 24 µm catalog); 36% of our galaxies have spectroscopic redshifts (mostly at z 10 11M) hyperluminous galaxies ( Lir > 10 12 L) at redshifts z ~ 2-3. Massive star-forming galaxies at redshifts 2 500 M yr _1) , and some of them are able to construct a mass of ~ 10 10–10 11 M in a single burst lifetime (~ 0.01–0.1 Gyr). At lower redshifts z ~ 2, massive star-forming galaxies are also present but appear to be building their stars on long timescales, either quiescently or in multiple modest burstlike episodes. At redshifts z ~ 1-2, the ability of the burstlike mode to produce entire galaxies in asingle event is limited to some lower ( M < 7 X 10 10 M) mass systems, and it is basically negligible at z < 1. Our results support a scenario in which star formation activity is differential with assembled stellar mass and redshift, and where the relative importance of the burstlike mode proceeds in a downsizing way from high to low redshift