The Mass-Independence of Specific Star Formation Rates in Galactic Disks

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 $B/T\lesssim0.6$, generally consistent with a pure-disk control sample ($\beta_{\rm control}=-0.05\pm0.04$). 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 $d\log M_*/dt$ 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.