Linking Stellar Mass and Star Formation in<i>Spitzer</i>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 m background in the GOODS/CDFS. Our identified sample consists of 747 Spitzer MIPS 24 m objects and includes $94% of all the 24 m sources in the GOODS-South field that have fluxes S (24 m) > 83 Jy (the $80% completeness limit of the Spitzer/GTO 24 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 jdzj jz spec z phot j/(1 z spec ) 0:02. We find that MIPS 24 m galaxies span the redshift range z $ 0 4 and that a substantial fraction (28%) lie at high redshifts z k 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 m galaxy population ranges from sources with intermediate luminosities (10 10 L < L IR < 10 11 L ) and low-to-intermediate assembled stellar masses (10 9 M P M P 10 11 M ) at z P 0:8, to massive (M k 10 11 M ) hyperluminous galaxies (L IR > 10 12 L ) at redshifts z $ 2 3. Massive star-forming galaxies at redshifts 2 P z P 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 %10 10 -10 11 M in a single burst lifetime ($0.01-0.1 Gyr). At lower redshifts z P 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 P 7 ; 10 10 M ) mass systems, and it is basically negligible at z P 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.