The coalescence of invariant manifolds and the spiral structure of barred galaxies

TL;DR

This paper demonstrates that invariant manifolds of all unstable periodic orbit families in a barred galaxy contribute to the formation and support of spiral structures, with manifolds causing stickiness that sustains long-lived spirals.

Contribution

It extends previous work by showing all unstable orbit families' manifolds support spiral structure, emphasizing their collective role in galaxy morphology.

Findings

Invariant manifolds support long-lived spiral arms.

Manifolds of all unstable orbit families contribute to spiral structure.

Stickiness of manifolds slows chaotic escape, maintaining spirals.

Abstract

In a previous paper (Voglis et al. 2006a, paper I) we demonstrated that, in a rotating galaxy with a strong bar, the unstable asymptotic manifolds of the short period family of unstable periodic orbits around the Lagrangian points L$_1$ or L$_2$ create correlations among the apocentric positions of many chaotic orbits, thus supporting a {\it spiral} structure beyond the bar. In the present paper we present evidence that the unstable manifolds of {\it all} the families of unstable periodic orbits near and beyond corotation contribute to the same phenomenon. Our results refer to a N-Body simulation, a number of drawbacks of which, as well as the reasons why these do not significantly affect the main results, are discussed. We explain the dynamical importance of the invariant manifolds as due to the fact that they produce a phenomenon of `stickiness' slowing down the rate of chaotic escape in an otherwise non-compact region of the phase space. We find a stickiness time of order 100 dynamical periods, which is sufficient to support a long-living spiral structure. Manifolds of different families become important at different ranges of values of the Jacobi constant. The projections of the manifolds of all the different families in the configuration space produce a pattern due to the `coalescence' of the invariant manifolds. This follows closely the maxima of the observed $m=2$ component near and beyond corotation. Thus, the manifolds support both the outer edge of the bar and the spiral arms.