A new mechanism of ring formation in barred galaxies

In this paper we establish the possibility of a new mechanism for the formation of ring-like structures in barred galaxies. We study the stability of periodic orbits with respect to perturbations perpendicular (vertical stability) or tangent (plane stability) to the plane of motion and we find that matter can be trapped in the case of a relatively strong and fast bar, in a narrow annular region around the bar. The periodic orbits inside the annulus are totally stable—i.e., both vertically and plane. The region outside the annulus is depleted of matter by the action of vertical or plane instability. The resulting ring-like structure has the geometrical characteristics of the true inner rings observed in barred galaxies. In this way the formation of inner rings seems to be a result of combined action ofvertical and plane instability. A similar study of stability in the outer parts of the Galaxy, where outer rings are observed, shows that the above mechanism is absent there.