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Register a new userThe Mass-Independence of Specific Star Formation Rates in Galactic Disks
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Louis E. Abramson
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The slope of the star formation rate/stellar mass relation (the SFR Main Sequence; ${rm SFR}-M_*$) is not quite unity: specific star formation rates $({rm SFR}/M_*)$ are weakly-but-significantly anti-correlated with $M_*$. Here we demonstrate that this trend may simply reflect the well-known increase in bulge mass-fractions -- portions of a galaxy not forming stars -- with $M_*$. Using a large set of bulge/disk decompositions and SFR estimates derived from the Sloan Digital Sky Survey, we show that re-normalizing SFR by disk stellar mass $({rm sSFR_{rm disk}equiv SFR}/M_{*,{rm disk}})$ reduces the $M_*$-dependence of SF efficiency by $sim0.25$ dex per dex, erasing it entirely in some subsamples. Quantitatively, we find $log {rm sSFR_{disk}}-log M_*$ to have a slope $beta_{rm disk}in[-0.20,0.00]pm0.02$ (depending on SFR estimator and Main Sequence definition) for star-forming galaxies with $M_*geq10^{10}M_{odot}$ and bulge mass-fractions $B/Tlesssim0.6$, generally consistent with a pure-disk control sample ($beta_{rm control}=-0.05pm0.04$). That $langle{rm SFR}/M_{*,{rm disk}}rangle$ is (largely) independent of host mass for star-forming disks has strong implications for aspects of galaxy evolution inferred from any ${rm SFR}-M_*$ relation, including: manifestations of mass quenching (bulge growth), factors shaping the star-forming stellar mass function (uniform $dlog M_*/dt$ for low-mass, disk-dominated galaxies), and diversity in star formation histories (dispersion in ${rm SFR}(M_*,t)$). Our results emphasize the need to treat galaxies as composite systems -- not integrated masses -- in observational and theoretical work.
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