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Astron. Astrophys. 328, 531-543

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Collisional dynamics of the Milky Way

Evgeny Griv1,2, Michael Gedalin1, and Chi Yuan2

1Department of Physics, Ben-Gurion University of the Negev, P.O. Box 653, Beersheva 84104, Israel
2Institute of Astronomy & Astrophysics, Academia Sinica, P.O. Box 1-87, Taipei 11529, Taiwan

Received 28 October 1996 / Accepted 23 April 1997


The effect of gravitational (elastic) encounters between stars and giant molecular clouds on the stability of small-amplitude perturbations of the Milky Way's self-gravitating disk is considered, using the exact Landau (Fokker-Planck type) collision integral, and compared with the results obtained by Griv & Peter (1996), who used the simple phenomenological Bhatnagar-Gross-Krook (Bhatnagar et al. 1954) collisional model. The present analysis is carried out for the case of a spatially inhomogeneous, highly flattened system, i.e., an inhomogeneous system in which the thickness is very small in comparison with the disk's radial extension. According to observations (Grivnev & Fridman 1990), the dynamics of a system with rare, $\kappa^2 \gg \nu_c^2$ (and weak, $\omega^2 \gg
\nu_c^2$), interparticle encounters is considered, where $\kappa$ is the epicyclic frequency, $\omega$ is the frequency of excited waves, and $\nu_c \sim 10^{-9}$ yr-1 is the effective frequency of star-cloud encounters. The evolution of the stellar distribution is determined primarily by interactions with collective modes of oscillations - gravitational Jeans-type and gradient-dissipative modes - rather than by ordinary (``close") star-cloud encounters. On the basis of a local kinetic theory, it is shown that the Landau integral and the Bhatnagar et al. model give practically identical results in the case of perturbations with the wavelength $\lambda$ that is comparable to the mean epicyclic radius of stars $\rho$, that is, in the case of the most dangerous, in the sense of the loss of stability, gravitational Jeans-type perturbations. The models, however, have essentially different qualitative and quantitative behaviors in the extreme limits of long-wavelength perturbations, $(\pi \rho /\lambda )^2 \ll 1$, and of short-wavelength perturbations, $(\pi \rho /\lambda )^2 \gg 1$. Certain observational implications of the present theory are discussed.

Key words: galaxies: interactions - galaxies: kinematics and dynamics - ISM: clouds

Send offprint requests to: Dr. E. Griv at Beersheva

© European Southern Observatory (ESO) 1997

Online publication: November 24, 1997
Last change: March 26, 1998