Formation of asymmetric arms in barred galaxies

TL;DR

This paper proposes a dynamical mechanism involving invariant manifolds around unstable equilibrium points to explain the observed asymmetry in spiral arms of barred galaxies, linking asymmetries to central mass distribution.

Contribution

It introduces a novel explanation for arm asymmetry in barred galaxies based on invariant manifolds and central mass distribution effects.

Findings

Asymmetric invariant manifolds lead to one well-defined and one ragged arm.

Mass distribution asymmetries cause differences in manifold extension and arm strength.

The model explains observed arm asymmetries in barred galaxies.

Abstract

We establish a dynamical mechanism to explain the origin of the asymmetry between the arms observed in some barred disk galaxies, where one of the two arms emanating from the bar ends is very well defined, while the second one displays a ragged structure, extending between its ridge and the bar. To this purpose, we study the invariant manifolds associated to the Lyapunov periodic orbits around the unstable equilibrium points at the ends of the bar. Matter from the galaxy center is transported along these manifolds to the periphery, forming this way the spiral arms that emanate from the bar ends. If the mass distribution in the galaxy center is not homogeneous, because of an asymmetric bar with one side stronger than the other, or because of a non-centered bulge, the dynamics about the two unstable Lagrange points at the ends of the bar will not be symmetric as well. One of their invariant manifolds becomes more extended than the other, enclosing a smaller section and the escaping orbits on it are fewer and dispersed in a wider region. The result is a weaker arm, and more ragged than the one at the other end of the bar.