Orbits in galactic bars

To understand the dynamics of a galactic bar, one must first understand the dynamics of the orbits that constitute it. For many years, it was the x1 family of periodic orbits that were considered as the orbital backbone of bars. Such orbits are simple and mainly stable, and they have an elliptical-like shape, elongated along the major axis of the bar, all important assets for obits that are the building blocks of bars. Lately, however, large numbers of higher multiplicity orbits have been found in the bar region of N-body bars. It was thus suggested that it could be them, and not the x1, that are the backbone of the bar. Contrary to the x1 tree of periodic orbits — which has been studied in depth including characteristic diagrams, surfaces of section and stability diagrams — hardly any properties of the higher multiplicity orbits in realistic N-body bar potentials have been so far studied. In this talk, I will describe our recent efforts to fill this gap and what we learnt about the properties of the higher multiplicity orbits. I will use these to discuss and compare the two alternatives, as well as their respective implicationson the bar properties. I will end with some suggestions of how to use Gaia data to get further constraints on this dilemma.