Two-fluid gravitational instabilities in a galactic disk

Jog, Chanda J. ; Solomon, P. M. (1984) Two-fluid gravitational instabilities in a galactic disk Astrophysical Journal, 276 . pp. 114-126. ISSN 0004-637X

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Official URL: http://articles.adsabs.harvard.edu/cgi-bin/nph-iar...

Related URL: http://dx.doi.org/10.1086/161597

Abstract

We forlulate and solve the hydrodynamic equations describing an azimuthally symmetric galactic disk as a. two-fluid system. The stars and the gas are treated as two different isothermal fluids of different velocity dispersions (Cs >> Cg ), which interact gravitationally with each other. The disk is supported by rotation and random motion. The formulation of the equations closely follows the one-fluid treatment by Toomre. We solve the lineraized perturbation equations by the method of modes, and study the stability of the galactic disk agaInst the growth of axisymmetric two-fluid gravitational instabilities. We find that even when both the fluids in a two-fluid system are separately stable, the joint two-fluid system, because of the gravitational interaction between the two fluids, may be unstable. The ratio of the gas contnbution to the stellar contribution toward the formation of two-fluid instabilities is substantially greater than μgs, the ratio of their respective surface densities; this is due to the lower gas velocity dispersion as compared to the stellar velocity dispersion (Cg << Cs). The two contributions are comparable when the gas fraction (μgs) IS only ~ 0.10-0.25. Therefore, a galactic disk is a meaningful two-fluid system even when the gas constitutes only 1O %-20 % of the total surface density. The ratio of the amplitude in the gas to the amplItude In the stars is an increasIng function of the wavenumber of the two-fluid perturbation. The wavelength and the time of growth of a typical two-fluid instability in the inner galaxy, for μgs = 0.1-0.2, and ~2-3 kpc and ~2-4 × 107 years, respectively, and each of these contains gas of mass 4 × 107-108 M. The two-fluid analysis presented here is applocable to any general disk galaxy consisting of stars and gas.

Item Type:Article
Source:Copyright of this article belongs to American Astronomical Society.
ID Code:14249
Deposited On:12 Nov 2010 08:45
Last Modified:16 May 2016 23:15

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