Global stability of disk-bulge systems: Spiral structure of disk galaxies |
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Authors: | Ashok Ambastha Ram K Varma |
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Institution: | (1) Physical Research Laboratory, 380009 Navrangpura, Ahmedabad |
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Abstract: | The spiral arms of disk galaxies are very sensitive to various morphological properties, such as, the gas content, the disk-to-bulge
ratioetc. Here, the stability of self-gravitating annular disks surrounding the central rigid bulge component has been studied in
order to explain the transition from the tight spiral arms in Sa galaxies to rather open patterns in Sc galaxies as the central
amorphous component diminishes. Smooth spiral patterns are found associated with the dominant (or the fastest growing) modes
of the system. When the disk-to-bulge mass ratio is small, a tight pattern results restricted to the inner regions of the
disk. This pattern opens up and occupies larger disk areas as the disk component becomes comparable to the bulge. It is found
here that the ‘explosive’ instabilities of the global density waves do not occur in the presence of a massive bulge. The growth-rates
of the eigen-modes decrease as the disk-to-bulge mass ratio decreases. It is also found that unstable modes of the annular
disk can be suppressed by increasing the thermal pressure sufficiently. |
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Keywords: | global density waves spiral galaxies disk-bulge systems |
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