TwoFluid Gravitational Instabilities in a Galactic Disk.
Abstract
We formulate and solve the hydrodynamic equations describing the galactic disk as a twofluid system where the stars and the gas are treated as two different isothermal fluids of different velocity dispersion (C(,1) >> C(,g)), which interact gravitationally with each other. The disk is supported by rotation and random motion. We treat the local, axisymmetric case. The formulation of the equations closely follows the onefluid treatment by Toomre (1964). We solve the linearised perturbation equations by the method of modes. We thus study the stability of the galactic disk against the growth of twofluid gravitational instabilities. We find that even when both the fluids in a two fluid system are separately stable; the joint twofluid system, due to the gravitational interaction between the two fluids in a twofluid system, may be unstable. The wavelength and the time of growth of a typical twofluid instability in the inner galaxy are (TURN) 23 kpc and (TURN) 24 x 10('7) years respectively. This may explain the observed patchy nature on this scale of the galactic morphology. The ratio of the gas contribution to the stellar contribution to the formation of twofluid instabilities is substantially greater than (mu)(,g)/(mu)(,s). Therefore, the galaxy is a meaningful twofluid system even when (mu)(,g)/(mu)(,s) is only (TURN) 0.10.2. The existence of even a small fraction of the total disk surface density in a cold fluid (that is, the gas) makes it much harder to stabilize the entire twofluid disk. At higher effective gas density resulting from the growth of a twofluid instability, the gas may become unstableeven when originally, the gas by itself is stable. We can thus explain the observations of the clumpy yet stable gas in the ISM. The wavelength of such a typical (induced) gas instability in the inner galaxy is (TURN) 400500 pc and it contains gas of mass (TURN) 12 x 10('7)M(,(CIRCLE)). The twofluid analysis presented here is applicable to any general disk galaxy consisting of stars and gas.
 Publication:

Ph.D. Thesis
 Pub Date:
 1982
 Bibcode:
 1982PhDT.........1J
 Keywords:

 Physics: Astronomy and Astrophysics;
 Disk Galaxies;
 Flow Stability;
 Gravitation;
 Hydrodynamic Equations;
 Galactic Rotation;
 Gas Density;
 Linear Equations;
 Perturbation;
 Wavelengths;
 Astronomy