The Gas-Star Formation Cycle in Nearby Star-Forming Galaxies I. Assessment of Multi-scale Variations

TL;DR

This study introduces a method to analyze how the spatial distribution of molecular gas and star formation varies across different scales in nearby galaxies, revealing a significant quiescent gas reservoir at small scales that diminishes at coarser resolutions.

Contribution

The paper presents a new, simple method to quantify scale dependence of molecular gas and star formation distributions in galaxies, bridging global and cloud-scale observations.

Findings

Quiescent molecular gas is prominent at 140pc resolution.

Morphological features become indistinct beyond 1kpc resolution.

Degradation of resolution causes rapid disappearance of quiescent gas features.

Abstract

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.