Gas flow in barred potentials II. Bar Driven Spiral Arms

TL;DR

This paper explains how spiral arms in barred galaxies are formed by kinematic density waves caused by librations around specific closed orbits, emphasizing the importance of orbital structure over simple potential estimates.

Contribution

It demonstrates that spiral arms are driven by librations around ballistic closed orbits, refining the understanding of bar strength and spiral arm formation in galactic dynamics.

Findings

Spiral arms can be modeled as kinematic density waves from librations.

Orbital structure determines bar strength classification.

Librations around appropriate closed orbits explain spiral arm formation.

Abstract

Spiral arms that emerge from the ends of a galactic bar are important in interpreting observations of our and external galaxies. It is therefore important to understand the physical mechanism that causes them. We find that these spiral arms can be understood as kinematic density waves generated by librations around underlying ballistic closed orbits. This is even true in the case of a strong bar, provided the librations are around the appropriate closed orbits and not around the circular orbits that form the basis of the epicycle approximation. An important consequence is that it is a potential's orbital structure that determines whether a bar should be classified as weak or strong, and not crude estimates of the potential's deviation from axisymmetry.