2023MNRAS.525.3287M

We study the periodic enhancement of either trailing or leading segments of the resonance elliptical rings in the dynamical model of the Galaxy which reproduces distributions of observed velocities derived from Gaia DR3 (EDR3) data along the Galactocentric distance. The model disc forms a nuclear ring, an inner combined ring, and outer resonance rings R₁ and R₂. The backbone of the inner combined ring is banana-type orbits around the Lagrange equilibrium points L₄ and L₅. Orbits associated with the unstable equilibrium points L₁ and L₂ also support the inner ring. We have found the changes of the morphology of the inner ring with a period of P = 0.57 ± 0.02 Gyr, which is close to the period of revolution along the long-period orbits around the points L₄ and L₅. A possible explanation of these morphological changes is the formation of an overdensity which then begins circulating along the closed contour. In the region of the Outer Lindblad Resonance (OLR), we have found the changes of the morphology of the outer rings with a period of P = 2.0 ± 0.1 Gyr. Probably, the morphological changes of the outer rings are due to the orbits trapped by the OLR. These orbits exhibit librations of the direction of orbital elongation with respect to the minor axis of the bar as well as the long-term variations in the stellar angular momentum, energy, average radius of the orbit, and eccentricity. Among many librating orbits, we discovered orbits with the libration period of P = 1.91 ± 0.01 Gyr, which may cause the morphological changes of the outer rings.