On the gravitational stability of the interstellar medium in spiral arms
Abstract
The dynamics of self-gravitating gaseous density perturbations in the tightly wound spiral-arm model of the forcing potential (Brown 1969) is investigated. The cloud ensemble of the cold phase of the interstellar medium is regarded as a collisional gas whose 'molecules' are the clouds themselves, and the dynamical evolution of density perturbations is calculated assuming this gas obeys a polytropic pressure-density law. It is suggested that the formation of extreme Population I objects found predominantly along the arms of Sb/Sc systems is regulated by an overall energy balance in the cloud fluid. A single parameter is found which appears to characterize the flow's gravitational response to perturbations; this spiral parameter, Q-sub-sp, differs from the Toomre Q parameter by a factor of the square root of the density enhancement at the shock. It is necessary but not sufficient that Q-sub-sp be less than unity for significant growth to occur.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 1985
- DOI:
- 10.1086/163503
- Bibcode:
- 1985ApJ...297...61B
- Keywords:
-
- Dynamic Stability;
- Galactic Evolution;
- Galactic Structure;
- Gravitational Constant;
- Interstellar Space;
- Spiral Galaxies;
- Astronomical Models;
- Gravitational Collapse;
- Astrophysics