2017ApJ...837..137M

The observationally derived Kennicutt-Schmidt (KS) relation slopes differ from study to study, ranging from sublinear to superlinear. We investigate the KS-relation variety (slope and normalization) as a function of integrated intensity ratio, R₃₁ = CO(J = 3-2)/CO(J = 1-0) using spatially resolved CO(J = 1-0), CO(J = 3-2), H i, Hα, and 24 µm data of three nearby spiral galaxies (NGC 3627, NGC 5055, and M83). We find that (1) the slopes for each subsample with a fixed R₃₁ are shallower, but the slope for all data sets combined becomes steeper, (2) normalizations for high R₃₁ subsamples tend to be high, (3) R₃₁ correlates with star formation efficiency, therefore the KS relation depends on the distribution in R₃₁-Σ_gas space of the samples: no Σ_gas dependence of R₃₁ results in a linear slope of the KS relation, whereas a positive correlation between Σ_gas and R₃₁ results in a superlinear slope of the KS relation, and (4) R₃₁-Σ_gas distributions are different from galaxy to galaxy and within a galaxy: galaxies with prominent galactic structure tend to have large R₃₁ and Σ_gas. Our results suggest that the formation efficiency of a star-forming cloud from molecular gas is different among galaxies as well as within a galaxy, and it is one of the key factors inducing the variety in galactic KS relation.