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; SFR-M-*) is not quite unity: specific star formation rates (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 (sSFR(disk) SFR/M*, disk) reduces the M-* dependence of SF efficiency by similar to 0.25 dex per dex, erasing it entirely in some subsamples. Quantitatively, we find log sSFR(disk)-log M-* to have a slope beta(disk) is an element of [-0.20, 0.00] +/- 0.02 (depending on the SFR estimator and Main Sequence definition) for star-forming galaxies with M-* >= 10(10) M-circle dot and bulge mass-fractions B/T less than or similar to 0.6, generally consistent with a pure-disk control sample (beta(control) = -0.05 +/- 0.04). That ( SFR/M-*,M- disk ) is (largely) independent of host mass for star-forming disks has strong implications for aspects of galaxy evolution inferred from any 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 SFR(M-*, t)). Our results emphasize the need to treat galaxies as composite systems-not integrated masses-in observational and theoretical work.