2014ApJ...787...68S

We use the observed radial profiles of the mass surface densities of total, Σ_g, and molecular, Σ_H2, gas, rotation velocity, and star formation rate (SFR) surface density, Σ_sfr, of the molecular-rich (Σ_H2 ≥ Σ_HI/2) regions of 16 nearby disk galaxies to test several star formation (SF) laws: a "Kennicutt-Schmidt (K-S)" law, Σ_sfr = A_gΣ_g,2^1.5; a "Constant Molecular" law, Σ_sfr= A_H2Σ_H2, 2_; the turbulence-regulated laws of Krumholz & McKee (KM05) and Krumholz, McKee, & Tumlinson (KMT09); a "Gas-Ω" law, Σ_sfr= B_ΩΣg_Ω; and a shear-driven "giant molecular cloud (GMC) Collision" law, Σ_sfr= B_CCΣ_gΩ(1-0.7β), where β ≡ d ln v_circ/d ln r. If allowed one free normalization parameter for each galaxy, these laws predict the SFR with rms errors of factors of 1.4-1.8. If a single normalization parameter is used by each law for the entire galaxy sample, then rms errors range from factors of 1.5-2.1. Although the Constant Molecular law gives the smallest rms errors, the improvement over the KMT, K-S, and GMC Collision laws is not especially significant, particularly given the different observational inputs that the laws utilize and the scope of included physics, which ranges from empirical relations to detailed treatment of interstellar medium processes. We next search for systematic variation of SF law parameters with local and global galactic dynamical properties of disk shear rate (related to β), rotation speed, and presence of a bar. We demonstrate with high significance that higher shear rates enhance SF efficiency per local orbital time. Such a trend is expected if GMC collisions play an important role in SF, while an opposite trend would be expected if the development of disk gravitational instabilities is the controlling physics.