Orbits in elementary, power-law galaxy bars: 1. Occurence and role of single loops

TL;DR

This paper investigates simple closed loop orbits in galaxy bars using analytic and numerical methods, revealing their rarity in self-gravitating bars and implications for bar longevity and structure.

Contribution

It introduces a detailed analysis of simple loop orbits in power-law galaxy potentials, highlighting their limited occurrence and role in supporting bar structures.

Findings

Simple loop orbits are rare in self-gravitating bars.

Librating orbit companions support annular bars.

Complex orbits are more common in realistic, three-dimensional bars.

Abstract

Orbits in galaxy bars are generally complex, but simple closed loop orbits play an important role in our conceptual understanding of bars. Such orbits are found in some well-studied potentials, provide a simple model of the bar in themselves, and may generate complex orbit families. The precessing, power ellipse (p-ellipse) orbit approximation provides accurate analytic orbit orbits in symmetric galaxy potentials. It remains useful for finding and fitting simple loop orbits in the frame of a rotating bar with bar-like and symmetric power-law potentials. Second order perturbation theory yields two or fewer simple loop solutions in these potentials. Numerical integrations in the parameter space neighborhood of perturbation solutions reveal zero or one ac- tual loops in a range of such potentials with rising rotation curves. These loops are embedded in a small parameter region of similar, but librating orbits, which have a subharmonic frequency superimposed on the basic loop. These loops and their librat- ing companions support annular bars. Solid bars can be produced in more complex potentials, as shown by an example with power-law indices varying with radius. The power-law potentials can be viewed as the elementary constituents of more complex potentials. Numerical integrations also reveal interesting classes of orbits with multiple loops. In two-dimensional, self-gravitating bars, with power-law potentials, single loop orbits are very rare. This result suggests that gas bars or oval distortions are unlikely to be long-lived, and that complex orbits or three-dimensional structure must support self-gravitating stellar bars.