Linking stellar mass and star formation in Spitzer MIPS 24 mu m galaxies

We present deep K-s <21: 5 (Vega) identifications, redshifts, and stellar masses for most of the sources composing the bulk of the 24 mu m background in the GOODS/CDFS. Our identified sample consists of 747 Spitzer MIPS 24 mu m objects and includes similar to 94% of all the 24 mu m sources in the GOODS-South field that have fluxes S-nu( 24 mu m) > 83 mu Jy ( the similar to 80% completeness limit of the Spitzer/GTO 24 mu m 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 vertical bar dz vertical bar = vertical bar Z(spec) - Z(phot)vertical bar/( 1 + Z(spec)) 0.02. We find that MIPS 24 mu m galaxies span the redshift range z similar to 0 4 and that a substantial fraction (28%) lie at high redshifts z greater than or similar to 1.5. We determine the existence of a bump in the redshift distribution at z similar to 1: 9, indicating the presence of a significant population of galaxies with PAH emission at these redshifts. The 24 mu m galaxy population ranges from sources with intermediate luminosities (10(10) L-circle dot <L-IR <10(11) L-circle dot) and low-to-intermediate assembled stellar masses (10(9) M circle dot M less than or similar to 10(11) M-circle dot) at z P 0: 8, to massive ( M less than or similar to 10(11) M-circle dot) hyperluminous galaxies (L-IR > 10(12) L-circle dot) at redshifts z less than or similar to 2-3. Massive star-forming galaxies at redshifts 2 less than or similar to z less than or similar to 3 are characterized by very high star formation rates (SFR > 500 M-circle dot yr(-1)), and some of them are able to construct a mass of approximate to 10(10) - 10(11) M-circle dot in a single burst lifetime ( similar to 0.01 - 0.1 Gyr). At lower redshifts z less than or similar to 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 similar to 1 2, the ability of the burstlike mode to produce entire galaxies in a single event is limited to some lower ( M less than or similar to 7; 10(10) M-circle dot) mass systems, and it is basically negligible at z less than or similar to 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