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 ∼0.25 dex per dex, erasing it entirely in some subsamples. Quantitatively, we find log sSFR disk –log M * to have a slope β disk ∈ [ − 0.20, 0.00] ± 0.02 (depending on the SFR estimator and Main Sequence definition) for star-forming galaxies with M * ⩾ 10 10 M ☉ and bulge mass-fractions B / T ≲ 0.6, generally consistent with a pure-disk control sample (β control = −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 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.