A Turnover in the Galaxy Main Sequence of Star Formation at M_* ~ 10^(10) M_☉ for Redshifts z < 1.3

Abstract

The relationship between galaxy star formation rates (SFRs) and stellar masses (M_*) is reexamined using a mass-selected sample of ~62,000 star-forming galaxies at z ≤ 1.3 in the COSMOS 2 deg^2 field. Using new far-infrared photometry from Herschel-PACS and SPIRE and Spitzer-MIPS 24 μm, along with derived infrared luminosities from the NRK method based on galaxies' locations in the restframe color-color diagram (NUV – r) versus (r – K), we are able to more accurately determine total SFRs for our complete sample. At all redshifts, the relationship between median SFR and M_* follows a power law at low stellar masses, and flattens to nearly constant SFR at high stellar masses. We describe a new parameterization that provides the best fit to the main sequence and characterizes the low mass power-law slope, turnover mass, and overall scaling. The turnover in the main sequence occurs at a characteristic mass of about M_0 ~ 10^(10) M_☉ at all redshifts. The low mass power-law slope ranges from 0.9-1.3 and the overall scaling rises in SFR as a function of (1 + z)^(4.12±0.10). A broken power-law fit below and above the turnover mass gives relationships of SFR ∝ M_*^(0.88±0.06) below the turnover mass and SFR ∝ M_*^(0.27±0.04) above the turnover mass. Galaxies more massive than M_* ≳ 10^(10) M_☉ have a much lower average specific star formation rate (sSFR) than would be expected by simply extrapolating the traditional linear fit to the main sequence found for less massive galaxies.

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