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.
Categories: astro-ph.GA astro-ph.CO