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.
Volume 40 | Issue 3
Since January 2016, the Journal of Astrophysics and Astronomy has moved to Continuous Article Publishing (CAP) mode. This means that each accepted article is being published immediately online with DOI and article citation ID with starting page number 1. Articles are also visible in Web of Science immediately. All these have helped shorten the publication time and have improved the visibility of the articles.