Kennicutt-Schmidt Relation Variety and Star-forming Cloud Fraction

Abstract

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, R 31  = CO( J = 3–2)/CO( J = 1–0) using spatially resolved CO( J = 1–0), CO( J = 3–2), H i , H α, and 24 μ m data of three nearby spiral galaxies (NGC 3627, NGC 5055, and M83). We find that (1) the slopes for each subsample with a fixed R 31 are shallower, but the slope for all data sets combined becomes steeper, (2) normalizations for high R 31 subsamples tend to be high, (3) R 31 correlates with star formation efficiency, therefore the KS relation depends on the distribution in R 31 –Σ gas space of the samples: no Σ gas dependence of R 31 results in a linear slope of the KS relation, whereas a positive correlation between Σ gas and R 31 results in a superlinear slope of the KS relation, and (4) R 31 –Σ gas distributions are different from galaxy to galaxy and within a galaxy: galaxies with prominent galactic structure tend to have large R 31 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.