Multiple Gas Rings in SA Galaxies

Abstract

Rings are frequently observed in barred galaxies (e.g. Buta & Crocker 1993). Their origin is attributed to gas accumulation near resonances, which is due to a combination of the gravitational torques of a bar component acting on inelastically colliding clouds: outer rings are presumably associated with the Outer Lindblad Resonance (OLR), inner rings with the 4/1 resonance, and nuclear rings with the Inner Lindblad Resonance (ILR).However, multiple rings are also observed in normal non-barred, or not obviously barred galaxies (e.g. NGC 7217, Verdes-Montenegro et al. 1995) so that we may ask what is the mechanism of their formation. We argue in this contribution that even a very weak bar or a triaxial distortion in the galaxy center, which would hardly be identified from observations, leads to long-lasting ring-like distributions of gaseous clouds. The formation time-scales of rings associated with different resonances are similar in our model so that all types of rings can coexist in galaxies with a high degree of axial symmetry in their central parts, such as SA galaxies.