A Test of Star Formation Laws in Disk Galaxies

We use observations of the radial profiles of the mass surface density of total, Σg, and molecular, ΣH2, gas, rotation velocity and star formation rate surface density, Σsfr, of the molecular dominated regions of 12 disk galaxies from Leroy et al. to test several star formation laws: a “Kennicutt power law”, Σsfr = AgΣ 1.5 g,2; a “Constant molecular law”, Σsfr = AH2ΣH2,2; the “Turbulence-regulated laws” of Krumholz & McKee (KM) and Krumholz, McKee & Tumlinson (KMT), a “Kennicutt Ω law”, Σsfr = BΩΣgΩ; and a shear-driven “GMC collisions law”, Σsfr = BCCΣgΩ(1 − 0.7β), where β ≡ d ln vcirc/d ln r. We find the constant molecular law, KMT turbulence law and GMC collision law are the most accurate, with an rms error of a factor of 1.5 if the normalization constants are allowed to vary between galaxies. Of these three laws, the GMC collision law does not require a change in physics to account for the full range of star formation activity seen from normal galaxies to circumnuclear starbursts. A single global GMC collision law with BCC = 8.0× 10 , i.e. a gas consumption time of 20 orbital times for β = 0, yields an rms error of a factor of 1.8. Subject headings: stars: formation — galaxies: evolution