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

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.