Abstract
The relationship between galaxy star formation rates (SFR) and stellar masses
($M_\ast$) is re-examined using a mass-selected sample of $\sim$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
$\mu$m, along with derived infrared luminosities from the NRK method based on
galaxies' locations in the restframe color-color diagram $(NUV - r)$ vs. $(r -
K)$, we are able to more accurately determine total SFRs for our complete
sample. At all redshifts, the relationship between median $SFR$ and $M_\ast$
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 $M_0 10^10 M_ødot$ 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 $SFR M_*^0.88 \pm
0.06$ below the turnover mass and $SFR M_*^0.27 0.04$ above
the turnover mass. Galaxies more massive than $M_10^10\ M_\rm
ødot$ have on average, a much lower 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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