Linking stellar mass and star formation in Spitzer MIPS 24 μm 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 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 S_ν(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| = |z_spec - z_phot|/(1 + z_spec) = 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_⊙ < L_IR < 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 (L_IR > 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.