view Abstract Citations (78) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Local Interstellar Gasdynamical Stability and Substructure in Spiral Arms Balbus, Steven A. Abstract A rather general local dynamical stability analysis for interstellar gas flow through the spiral arm regions (or regions with similar oblique potentials) in disk galaxies is presented. The background flow may in principal be quite arbitrary as long as the gas orbits remain approximately circular; highly compressed flow is permitted. A polytropic single-fluid treatment is used. All perturbation wavenumbers in the plane of the disk are considered. The calculations reduce to the Goldreich-Lynden-Bell (GLB) swing amplifier in the limit of unit enhancement (i.e., no compression) in the spiral arm. Our results thus generalize both GLB and an earlier paper by Balbus and Cowie (BC) in which Jeans-like disturbances were analyzed. If the underlying galactic rotation curve is flat, then density compressions greater than a factor of 2 cause the background angular velocity locally to increase outward. This disrupts the classical swing amplification mechanism by destroying the consonance of the epicyclic and shear motion. This would tend to cause unsheared tightly wound disturbances to grow most rapidly. But opposing this tendency is a divergence swing amplifier: the underlying expansion of the flow, being nonisotropic in the disk plane, generally both separates and rotates wave crests in a perturbation. (Similar effects are associated with compression). If this induced rotation is in the same sense as the epicyclic motion, then amplification can occur. This process favors loosely wound, unwrapped perturbations. Growth rates for both the tightly and loosely wound arms can be comparable and depend upon the overall stability of the underlying flow: more sensitive flow favors tightly wrapped disturbances; less sensitive (but still responsive) flow favors loosely-wrapped wavelets. Because expanding flow tends to resist winding, the latter may be associated with periodic spurs often seen protruding from principal arms in spiral galaxies, particularly in the outer regions of the disk. Another possible generating mechanism for spurs or branches is that tightly-wound growing wave crests can be distorted by the tidal velocity field which is both expanding and changing shear direction near the arm. This depends upon departures from uniform expansion and is accordingly a higher order effect that can be treated via local analysis. These structures need not be periodically spaced along an arm. When the most rapidly growing tight and loose wavelets respond on a comparable time scale, nonlinear periodic structure may be expected to appear at the intersections of wave crests. We identify these with cloud complexes. Characteristic spacings and masses are in agreement with observations. A morphological progression-thickened arms with irregular structure, arms with periodic structure, highly branched or spurred arms-is suggested by the results, with the BC local stability parameter Q_sp_ (or its equivalent) determining the outcome. Publication: The Astrophysical Journal Pub Date: January 1988 DOI: 10.1086/165880 Bibcode: 1988ApJ...324...60B Keywords: Disk Galaxies; Dynamic Stability; Galactic Structure; Gas Dynamics; Interstellar Gas; Spiral Galaxies; Density Distribution; Euler Equations Of Motion; Perturbation Theory; Astrophysics; GALAXIES: STRUCTURE; HYDRODYNAMICS; INSTABILITIES full text sources ADS | data products SIMBAD (1) NED (1)