Linking Stellar Mass and Star Formation in Spitzer MIPS 24 µm Galaxies

Abstract

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_solar<L_IR<10¹¹ L_solar) and low-to-intermediate assembled stellar masses (10⁹ M_solar<~M<~10¹¹ M_solar) at z<~0.8, to massive (M>~10¹¹ M_solar) hyperluminous galaxies (L_IR>10¹² L_solar) 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_solar yr^-1), and some of them are able to construct a mass of ~10¹⁰-10¹¹ M_solar 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_solar) 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.