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

Abstract

Context. There is a clear 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. We aim to calculate the scales of regularity in the grand-design galaxy NGC 628 in terms of gravitational instability by using modern high-quality observational data and comparing them with scales of spatial regularity of the star-forming regions and molecular clouds in the spiral arms of the galaxy.Methods. We investigated two mechanisms of gravitational instability against radial and azimuthal perturbations in a multicomponent disk with a finite thickness. We obtained a map of the instability wavelength distribution and compared its median value with the typical scale of observed regularity.Results. The maps of instability parameters Q and S, which are related to the 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 had previously been observed for star-forming regions. Additionally, the distribution of the wavelength most unstable to azimuthal perturbations yields a median value of about 700 pc, which is close to the observed scale regularity. This latter finding helps resolve the discrepancy between theoretically predicted and observed scales of star-forming regions in terms of regularity.