Gravitational instability and spatial regularity of the gas clouds and young stellar population in spiral arms of NGC 628

TL;DR

This study reconciles the discrepancy between observed star-forming region spacings (~500 pc) and theoretical gravitational instability scales (>1 kpc) in NGC 628 by analyzing instability mechanisms and molecular cloud distributions.

Contribution

The paper demonstrates that the most unstable azimuthal wavelength (~700 pc) aligns with observed regularity, resolving previous scale discrepancies in spiral arms of NGC 628.

Findings

Median instability wavelength for azimuthal perturbations is about 700 pc.

Observed molecular cloud distribution shows regularity of 500-600 pc.

Gravitational instability regions correlate with recent star formation areas.

Abstract

Context. There is a contradiction between the characteristic spacings in observed regular chains of star-forming regions in the spiral arms of galaxies, ~500 pc, and the estimates of the wavelength of gravitational instability in them, >1 kpc. Aims. Calculating the scales of regularity in the grand-design galaxy NGC 628 in terms of gravitational instability, using modern high-quality observational data and comparison of them with scales of spatial regularity of the star-forming regions and molecular clouds in the spiral arms of the galaxy. Methods. We investigate two mechanisms of gravitational instability against radial and azimuthal perturbations in a multi-component disk with a finite thickness. We obtain a map of the instability wavelength distribution and compare its median value with the typical scale of observed regularity. Results. The maps of instability parameters Q and S, which are related to radial and azimuthal perturbations respectively, show a good alignment between gravitationally unstable regions and areas of recent star formation. By analyzing the distribution of giant molecular clouds along the spiral arms of NGC 628, we found a similar regularity of about 500-600 pc that was previously observed for starforming regions. Additionally, the distribution of the wavelength most unstable to azimuthal perturbations yielded a median value of about 700 pc, which is close to the observed scale regularity. This last finding resolved the discrepancy between the theoretically predicted and observed scales of star-forming regions' regularity.