The orbital content of bars: the origin of ‘non-x1-tree’, bar-supporting orbits

Abstract

ABSTRACT Recently, many orbital studies in barred galaxy potentials have revealed the existence of orbits that are not trapped around x1-tree orbits, but could be potentially appropriate building blocks for bars. These findings question the uniqueness of the x1 family as the standard paradigm of orbital motion in galactic bars. The main goal of this paper is to investigate the role that such orbits could play in shaping the morphology of bars. We trace the morphological patterns appearing in the face-on and edge-on views of the non-periodic orbits presented in these studies and we show that they are introduced in the system by second type (‘deuxième genre’) bifurcations of x1. For this purpose, we use a typical 3D Ferrers bar model and follow the radial and vertical bifurcations of the x1 family considered as being mul-periodic, with mul = 2, 3, 5. The variation of the stability indices of x1 in the mul = 2, 3 cases gives us also the 4- and 6-periodic orbits, respectively. We tabulate these orbits including all information necessary to assess their role as appropriate building blocks. We discuss their stability and their extent, as well as their size and morphological evolution, as a function of energy. We conclude that even the most important of the mul-periodic orbits presented in Tables 2–5 are less appropriate building blocks for bars than the families of the x1-tree at the same energy.