A Test of Star Formation Laws in Disk Galaxies
  We use observations of the radial profiles of the mass surface density of
total, Sigma_g, and molecular, Sigma_H2, gas, rotation velocity and star
formation rate surface density, Sigma_sfr, of the molecular dominated regions
of 12 disk galaxies from Leroy et al. to test several star formation laws: a
"Kennicutt-Schmidt power law", Sigma_sfr=A_g Sigma_{g,2}^{1.5}$; a "Constant
molecular law", Sigma_sfr = A_H2 Sigma_{H2,2}; the "Turbulence-regulated laws"
of Krumholz & McKee (KM) and Krumholz, McKee & Tumlinson (KMT), a "Gas-Omega
law", Sigma_sfr = B_Omega Sigma_g Omega; and a shear-driven "GMC collisions
law", Sigma_sfr = B_CC Sigma_g Omega (1 - 0.7beta), where beta is d ln v_circ /
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
B_CC=8.0x10^{-3}, i.e. a gas consumption time of 20 orbital times for beta=0,
yields an rms error of a factor of 1.8.
