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 (sSFRdisk ≡ SFR/M*, disk) reduces the M* dependence of SF efficiency by ∼0.25 dex per dex, erasing it entirely in some subsamples. Quantitatively, we find log sSFRdisk–log M* to have a slope βdisk ∈ [ − 0.20, 0.00] ± 0.02 (depending on the SFR estimator and Main Sequence definition) for star-forming galaxies with M* ⩾ 1010 M☉ and bulge mass-fractions B/T ≲ 0.6, generally consistent with a pure-disk control sample (βcontrol = −0.05 ± 0.04). That 〈SFR/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 dlog 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.