The mass-independence of specific star formation rates in galactic disks

Abstract

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 * ≥ 1010 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 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. © 2014. The American Astronomical Society. All rights reserved..