4.1. Orbits and resonances
First, let us recall the characteristics of orbits in an axisymmetric potential (r) in the plane z = 0. A circular orbit has an angular velocity ^{2} = 1/r d / dr. In linearizing the potential in the neighborhood of a circular orbit, the motion of any particle can be expressed in first order by an epicyclic oscillation, of frequency ,
The general orbit is therefore the combination of a circle and an epicycle, or a rosette, since there is no rational relation between the two periods.
The bar creates a bisymmetric gravitational potential, with a predominant Fourier component m = 2, which rotates in the galaxy with the pattern speed _{b}. There is a region in the plane where the pattern speed is equal to the frequency of rotation , and where particles do not make any revolution in the rotating frame. This is the resonance of corotation (cf figure 4).
In the rotating frame, the effective angular velocity of a particle is ' = - _{b}. There exists then regions in the galaxy where ' = /m, i. e. where the epicyclic orbits close themselves after m lobes. The corresponding stars are aligned with the perturbation and closely follow it; they interact with it always with the same sign, and resonate with it. These zones are the Lindblad resonances, sketched in Figure 4. According to the relative values of and in a realistic disk galaxy, and because the bar is a bisymmetric perturbation, the most important resonances are those for m = 2.
Periodic orbits in the bar rotating frame are orbits that close on themselves after one or more turns. Periodic orbits are the building blocks which determine the stellar distribution function, since they define families of trapped orbits around them. Trapped orbits are non-periodic, but oscillate about one periodic orbit, with a similar shape. The various families are (Contopoulos & Grosbol 1989):