The Gas-Star Formation Cycle in Nearby Star-forming Galaxies II.
  Resolved Distributions of CO and H$\alpha$ Emission for 49 PHANGS Galaxies
  The relative distribution of molecular gas and star formation in galaxies
gives insight into the physical processes and timescales of the cycle between
gas and stars. In this work, we track the relative spatial configuration of CO
and H$\alpha$ emission at high resolution in each of our galaxy targets, and
use these measurements to quantify the distributions of regions in different
evolutionary stages of star formation: from molecular gas without star
formation traced by H$\alpha$ to star-forming gas, and to HII regions. The
large sample, drawn from the Physics at High Angular resolution in Nearby
GalaxieS ALMA and narrowband H$\alpha$ (PHANGS-ALMA and PHANGS-H$\alpha$)
surveys, spans a wide range of stellar mass and morphological types, allowing
us to investigate the dependencies of the gas-star formation cycle on global
galaxy properties. At a resolution of 150 pc, the incidence of regions in
different stages shows a dependence on stellar mass and Hubble type of galaxies
over the radial range probed. Massive and/or earlier-type galaxies exhibit a
significant reservoir of molecular gas without star formation traced by
H$\alpha$, while lower-mass galaxies harbor substantial HII regions that may
have dispersed their birth clouds or formed from low-mass, more isolated
clouds. Galactic structures add a further layer of complexity to relative
distribution of CO and H$\alpha$ emission. Trends between galaxy properties and
distributions of gas traced by CO and H$\alpha$ are visible only when the
observed spatial scale is $\ll$ 500 pc, reflecting the critical resolution
requirement to distinguish stages of star formation process.
