Manifold spirals in barred galaxies with multiple pattern speeds

Abstract

In the manifold theory of spiral structure in barred galaxies, the usual assumption is that the spirals rotate with the same pattern speed as the bar. Here we generalize the manifold theory under the assumption that the spirals rotate with different pattern speed than the bar. More generally, we consider the case when one or more modes, represented by the potentials V_2, V_3, \ldots, co-exist in the galactic disc in addition to the bar's mode V_{bar}, but rotate with pattern speeds \Omega_2, \Omega_3, \ldots incommensurable between themselves and with \Omega_{bar}. Through a perturbative treatment (assuming that V_2,V_3... are small with respect to V_{bar}) we then show that the unstable Lagrangian points L_1, L_2 of the pure bar model (V_{bar},\Omega_{bar}) are `continued' in the full model as periodic orbits, when we have one extra pattern speed different from \Omega_{bar}, or as epicyclic `Lissajous-like' unstable orbits, when we have more than one extra pattern speeds. As an example we compute the generalized orbits GL_1, GL_2 and their manifolds in a Milky-way like model with bar and spiral pattern speeds assumed different. We find that the manifolds produce a time-varying morphology consisting of segments of spirals or `pseudorings'. These structures are repeated after a period equal to half the relative period of the imposed spirals with respect to the bar. Along one period, the manifold-induced time-varying structures are found to continuously support at least some part of the imposed spirals, except at short intervals around those times at which the relative phase of the imposed spirals with respect to the bar becomes equal to \pm\pi/2. A connection of these effects to the phenomenon of recurrent spirals is discussed.