The Gas-Star Formation Cycle in Nearby Star-forming Galaxies II. Resolved Distributions of CO and H$\alpha$ Emission for 49 PHANGS Galaxies

TL;DR

This study investigates the spatial relationship between molecular gas and star formation in 49 nearby galaxies, revealing how galaxy properties influence the distribution of different star formation stages at high resolution.

Contribution

It provides a detailed analysis of the resolved distributions of CO and H$oldsymbol{ extalpha}$ emission across diverse galaxy types, highlighting dependencies on galaxy mass, morphology, and spatial scale.

Findings

Massive galaxies have more molecular gas without star formation.

Lower-mass galaxies show more HII regions indicating recent star formation.

Spatial resolution below 500 pc is crucial to distinguish star formation stages.

Abstract

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.