Patricia Sanchez-Martin

CUD, Universidad de Zaragoza, Spain

Dynamical study of multiple arm formation in barred galaxies

Galaxies are characterized by the distribution of their arms, but the reason for multiple pairs of spiral arms is still unclear. This work focuses on the formation of multiple-armed barred galaxies due to the decrease of the bar pattern speed. We analyze the effect of this change in the invariant manifolds of the galactic model. These manifolds transport matter between the exterior and interior parts of the galaxy, creating visible structures in form of arms and rings.
From the planar Lyapunov orbits around the unstable equilibrium points at the ends of the bar emanate the normally hyperbolic invariant manifolds sketching the galaxy arms. These manifolds and their intersections define the structures in charge of the transport of matter. We examine the idea that the manifolds form multiple-armed galaxies owing to the detected decrease bar pattern speed: first, the bar develops a pair of arms determined by the current velocity of the bar through the associated invariant manifolds around the equilibrium points; the bar evolves along time decreasing its pattern speed; the former arms survive and now the bar creates another pair of arms due to the new pattern speed. This mechanism leads to two different pairs of arms with different pattern speed, frequently observed in N-body simulations and galaxy observations.

We obtain in addition a quantitative estimation of the performance of the model in a non-autonomous setting thanks to the orbits of particles captured by the invariant manifolds. In an autonomous model, these orbits follow the manifolds, but we observe that in a time varying potential the behaviour of these orbits depends on the slope of the decreasing pattern speed, and this fact is decisive in the apparition of separate pairs of arms rather than a continuous distribution of matter.

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Dynamical study of multiple arm formation in barred galaxies
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