The Gas-Star Formation Cycle in Nearby Star-Forming Galaxies I.
  Assessment of Multi-scale Variations
  The processes regulating star formation in galaxies are thought to act across
a hierarchy of spatial scales. To connect extragalactic star formation
relations from global and kpc-scale measurements to recent cloud-scale
resolution studies, we have developed a simple, robust method that quantifies
the scale dependence of the relative spatial distributions of molecular gas and
recent star formation. In this paper, we apply this method to eight galaxies
with roughly 1 arcsec resolution molecular gas imaging from the PHANGS-ALMA and
PAWS surveys that have matched resolution, high quality narrowband Halpha
imaging. At a common scale of 140pc, our massive (log(Mstar/Msun)=9.3-10.7),
normally star-forming (SFR/Msun/yr=0.3-5.9) galaxies exhibit a significant
reservoir of quiescent molecular gas not associated with star formation as
traced by Halpha emission. Galactic structures act as backbones for both
molecular and HII region distributions. As we degrade the spatial resolution,
the quiescent molecular gas disappears, with the most rapid changes occurring
for resolutions up to about 0.5kpc. As the resolution becomes poorer, the
morphological features become indistinct for spatial scales larger than about
1kpc. The method is a promising tool to search for relationships between the
quiescent or star-forming molecular reservoir and galaxy properties, but
requires a larger sample size to identify robust correlations between the
star-forming molecular gas fraction and global galaxy parameters.
