Analytical forms of chaotic spiral arms

We develop an analytical theory of chaotic spiral arms in galaxies. This is based on the Moser theory of invariant manifolds around unstable periodic orbits. We apply this theory to the chaotic spiral arms, which start from the neighbourhood of the Lagrangian points L-1 and L-2 at the end of the bar in a barred-spiral galaxy. The series representing the invariant manifolds starting at the Lagrangian points L-1, L-2, or unstable periodic orbits around L-1 and L-2, yield spiral patterns in the configuration space. These series converge in a domain around every Lagrangian point, called 'Moser domain', and represent the orbits that constitute the chaotic spiral arms. In fact, these orbits are not only along the invariant manifolds, but also in a domain surrounding the invariant manifolds. We show further that orbits starting outside the Moser domain but close to it converge to the boundary of the Moser domain, which acts as an attractor. These orbits stay for a long time close to the spiral arms before escaping to infinity.