The organization of cloud-scale gas density structure: high resolution
  CO vs. 3.6 $\mu$m brightness contrasts in nearby galaxies
  In this paper we examine the factors that shape the distribution of molecular
gas surface densities on the 150 pc scale across 67 morphologically diverse
star-forming galaxies in the PHANGS-ALMA CO (2-1) survey. Dividing each galaxy
into radial bins, we measure molecular gas surface density contrasts, defined
here as the ratio between a fixed high percentile of the CO distribution and a
fixed reference level in each bin. This reference level captures the level of
the faint CO floor that extends between bright filamentary features, while the
intensity level of the higher percentile probes the structures visually
associated with bright, dense ISM features like spiral arms, bars, and
filaments. We compare these contrasts to matched percentile-based measurements
of the 3.6 $\mu$m emission measured using Spitzer/IRAC imaging, which trace the
underlying stellar mass density. We find that the logarithms of CO contrasts on
150 pc scales are 3-4 times larger than, and positively correlated with, the
logarithms of 3.6 $\mu$m contrasts probing smooth non-axisymmetric stellar bar
and spiral structures. The correlation appears steeper than linear, consistent
with the compression of gas as it flows supersonically in response to
large-scale stellar structures, even in the presence of weak or flocculent
spiral arms. Stellar dynamical features appear to play an important role in
setting the cloud-scale gas density in our galaxies, with gas self-gravity
perhaps playing a weaker role in setting the 150 pc-scale distribution of gas
densities.
