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In this paper we examine the factors that shape the distribution of moleculargas surface densities on the 150 pc scale across 67 morphologically diversestar-forming galaxies in the PHANGS-ALMA CO (2-1) survey. Dividing each galaxyinto radial bins, we measure molecular gas surface density contrasts, definedhere as the ratio between a fixed high percentile of the CO distribution and afixed reference level in each bin. This reference level captures the level ofthe faint CO floor that extends between bright filamentary features, while theintensity level of the higher percentile probes the structures visuallyassociated with bright, dense ISM features like spiral arms, bars, andfilaments. We compare these contrasts to matched percentile-based measurementsof the 3.6 $\mu$m emission measured using Spitzer/IRAC imaging, which trace theunderlying stellar mass density. We find that the logarithms of CO contrasts on150 pc scales are 3-4 times larger than, and positively correlated with, thelogarithms of 3.6 $\mu$m contrasts probing smooth non-axisymmetric stellar barand spiral structures. The correlation appears steeper than linear, consistentwith the compression of gas as it flows supersonically in response tolarge-scale stellar structures, even in the presence of weak or flocculentspiral arms. Stellar dynamical features appear to play an important role insetting the cloud-scale gas density in our galaxies, with gas self-gravityperhaps playing a weaker role in setting the 150 pc-scale distribution of gasdensities.

The organization of cloud-scale gas density structure: high resolution CO vs. 3.6 $\mu$m brightness contrasts in nearby galaxies