Perturbed precessing ellipses as the building blocks of spiral arms in a barred galaxy with two pattern speeds

¹ Research Center for Astronomy and Applied Mathematics, Academy of Athens, Soranou Efessiou 4, 115 27 Athens, Greece
e-mail: mharsoul@academyofathens.gr; gcontop@academyofathens.gr
² Department of Mathematics, Tullio Levi-Civita, University of Padua, Via Trieste, 63, 35121 Padova, Italy
e-mail: cefthim@academyofathens.gr

Received: 18 May 2022
Accepted: 13 September 2022

Abstract

Observations and simulations of barred spiral galaxies have shown that, in general, the spiral arms rotate at a different pattern speed to that of the bar. The main conclusion from the bibliography is that the bar rotates faster than the spiral arms with a double or even a triple value of angular velocity. The theory that prevails in explaining the formation of the spiral arms in the case of a barred spiral galaxy with two pattern speeds is the manifold theory, where the orbits that support the spiral density wave are chaotic, and are related to the manifolds emanating from the Lagrangian points L₁ and L₂ at the end of the bar. In the present study, we consider an alternative scenario in the case where the bar rotates fast enough in comparison with the spiral arms and the bar potential can be considered as a perturbation of the spiral potential. In this case, the stable elliptical orbits that support the spiral density wave (in the case of grand design galaxies) are transformed into quasiperiodic orbits (or 2D tori) with a certain thickness. The superposition of these perturbed preccesing ellipses for all the energy levels of the Hamiltonian creates a slightly perturbed symmetrical spiral density wave.