Linking Stellar Mass and Star Formation in Spitzer MIPS 24 m Galaxies
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We present deep Ks < 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 |dz| = |zspec - zphot|/(1 + zspec) = 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 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 (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. 2006. The American Astronomical Society. All rights reserved.