We present deep Ks 21.5 (Vega) identifications, redshifts, and stellar masses for most of the sources composing the bulk of the 24 μm background in the GOODS/CDFS. Our identified sample consists of 747 Spitzer MIPS 24 μm objects and includes ~94% of all the 24 μm sources in the GOODS-South field that have fluxes Sν(24 μm) 83 μJy (the ~80% completeness limit of the Spitzer/GTO 24 μm catalog); 36% of our galaxies have spectroscopic redshifts (mostly at z 1.5), and the remaining ones have photometric redshifts of very good quality, with a median of |dz| = |zspec - zphot|/(1 + zspec) = 0.02. We find that MIPS 24 μm galaxies span the redshift range z ~ 0-4 and that a substantial fraction (28%) lie at high redshifts z 1.5. We determine the existence of a bump in the redshift distribution at z ~ 1.9, indicating the presence of a significant population of galaxies with PAH emission at these redshifts. The 24 μm galaxy population ranges from sources with intermediate luminosities (1010 L☉ LIR 1011 L☉) and low-to-intermediate assembled stellar masses (109 M☉ M 1011 M☉) at z 0.8, to massive (M 1011 M☉) hyperluminous galaxies (LIR 1012 L☉) at redshifts z ~ 2-3. Massive star-forming galaxies at redshifts 2 z 3 are characterized by very high star formation rates (SFR 500 M☉ yr-1), and some of them are able to construct a mass of ≈1010-1011 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 a single event is limited to some lower (M 7 × 1010 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 redshifts.
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