The Mass-Independence of Specific Star Formation Rates in Galactic Disks

Louis E. Abramson; Daniel D. Kelson; Alan Dressler; Bianca M. Poggianti; Michael D. Gladders; Augustus Oemler; Benedetta Vulcani

doi:10.1088/2041-8205/785/2/L36

paper

The Mass-Independence of Specific Star Formation Rates in Galactic Disks

arXiv:1402.7076 · doi:10.1088/2041-8205/785/2/L36

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 $(sSF R_{disk} \equiv SFR / M_{*, disk})$ reduces the $M_{*}$ -dependence of SF efficiency by $\sim 0.25$ dex per dex, erasing it entirely in some subsamples. Quantitatively, we find $lo g sSF R_{disk} - lo g M_{*}$ to have a slope $β_{disk} \in [- 0.20, 0.00] \pm 0.02$ (depending on SFR estimator and Main Sequence definition) for star-forming galaxies with $M_{*} \geq 1 0^{10} M_{⊙}$ and bulge mass-fractions $B / T ≲ 0.6$ , generally consistent with a pure-disk control sample ( $β_{control} = - 0.05 \pm 0.04$ ). 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 $d lo g M_{*} / d t$ 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.

6 pages, 2 figures; Accepted to ApJL March 2014. ArXiv version updated to reflect changes made during refereeing; results unchanged

The Mass-Independence of Specific Star Formation Rates in Galactic Disks

Abstract

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