A turnover in the galaxy main sequence of star formation at M _∗ ∼ 10¹⁰ M _⊙ for redshifts z < 1.3

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 deg² 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 10¹⁰ 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 below the turnover mass and above the turnover mass. Galaxies more massive than M _∗ ≳ 10¹⁰ 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.