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 sub-linear to super-linear. 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), HI, Ha and 24um data of three nearby spiral galaxies (NGC3627, NGC5055 and M83). We find that (1) the slopes for each subsample with a fixed R31 are shallower but the slope for all datasets combined becomes steeper, (2) normalizations for high R31 subsamples tend to be high, (3) R31 correlates with star-formation efficiency, thus the KS relation depends on the distribution in R31-Sigma_gas space of the samples: no Sigma_gas dependence of R31 results in a linear slope of the KS relation whereas a positive correlation between Sigma_gas and R31 results in a super-linear slope of the KS relation, and (4) R31-Sigma_gas distributions are different from galaxy to galaxy and within a galaxy: galaxies with prominent galactic structure tend to have large R31 and Sigma_gas. Our results suggest that the formation efficiency of star-forming cloud from molecular gas is different among galaxies as well as within a galaxy and is one of the key factors inducing the variety in galactic KS relation.

14 pages, 17 figures, 4 tables, accepted for publication in ApJ

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• The CO(3-2)/CO(1-0) luminosity line ratio in nearby star-forming galaxies and AGN from xCOLD GASS, BASS and SLUGS
• The effect of diffuse background on the spatially-resolved Schmidt relation in nearby spiral galaxies
• First Detection of the Molecular Cloud Population in the Extended Ultraviolet (XUV) Disk of M83
• The Arizona Molecular ISM Survey with the SMT: Variations in the CO(2-1)/CO(1-0) Line Ratio Across the Galaxy Population
• Measuring 60-pc-scale Star Formation Rate of the Nearby Seyfert Galaxy NGC 1068 with ALMA, HST, VLT/MUSE, and VLA