Outer dynamics and escapes in barred galaxies

Abstract

We study the dynamics of the outer parts of barred galaxies (beyond corotation). In this region there are several islands of stability around stable periodic orbits and chaotic orbits that eventually escape from the galaxy. We study the forms of the orbits (periodic and non-periodic) and the intersections of the orbits by a surface of section (x, x), i.e. y = 0, for fixed values of the Jacobi constant. The structure of phase space is characterized by the rotation curve, i.e. the rotation number of successive orbits with initial conditions along the x-axis. The non-escaping orbits have initial conditions inside a 'last KAM torus'. The escaping orbits either have a positive energy from the beginning, or they change their energy at irregular intervals of time, acquiring finally positive energy that leads to escape. The successive consequents (intersections) of such an orbit are along 'bell-type curves' (one curve for each value of the energy). On the surface of section we see also almost rectilinear 'rays' that are the images of the x-axis (i.e. y = y = 0). The lower parts of these rays represent escaping orbits. These escapes are much faster than the escapes produced by encounters of stars among themselves. We considered two models of galactic type and various values of the Jacobi constant. These models have the same basic behaviour as regards the non-escaping and escaping orbits. This is an indication that these phenomena are characteristic of the outer parts of generic rotating galaxies.