Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995nyasa.773..111h&link_type=abstract
Waves in Astrophysics, vol. Volume 773, p. 111-124
Astronomy and Astrophysics
Astrophysics
1
Galactic Structure, Disk Galaxies, Stellar Systems, Stellar Models, Thickness, Bending, Galactic Halos, Perturbation, Milky Way Galaxy, Circular Orbits, Galactic Rotation, Warpage
Scientific paper
Warping of disk galaxies is much more evident in the gas than in stars. The warp in the H1 layer of the Milky Way was detected form 21-cm observations in the 1950s. Then, in the 1970s, it became apparent that warps are a common feature of the H1 layers of external galaxies. Warps of H1 layers become strong only where they extend far beyond the optical image, and the optical evidence for the occurrence of warps in stellar disks is much less strong. Two of the mechanisms that have been proposed for explaining warps, an intergalactic wind and an infall of matter, are likely to affect gaseous disks more strongly than stellar disks. Other proposed mechanisms apply equally well to stellar as to gaseous disks. The Milky Way warp is much too large to be attributed to the tide currently being raised by any neighbor. This explanation is similarly unsatisfactory for the warps in many external galaxies. Both this and other possibilities require an understanding of the dynamics of bent disks and the waves that they can support. It is this dynamics, rather than the explanation of galactic warps, that is the topic of this paper. Massive dark halos may play a major role in galactic warps. The difficulty of detecting warps in stellar disks suggests that they are generally stable against bending perturbations. If dark halos are the major cause of the warping of the gas beyond the optical disk, it is unlikely that they would also help stabilize the stellar disk. The simplest possible dynamical model, and one that is still widely used even when coupled with more elaborate halo models, is that of a razor-thin flat disk in which matter rotates in circular orbits around a center. Random velocities in the plane, as there surely are in flat stellar systems, show that this model can have only a limited range of validity. Presumably the consequence of the bending instability that results if a system becomes too flat is that it thickens. Also, inhomogeneities play a major role; bending modes in inhomogeneous disks can remain unstable at very long wavelengths. There remains the question of whether it is possible to construct a simplified model of a flattened stellar system that includes the major phenomena of ordered and random motions, small but finite thickness, and inhomogeneities.
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