Can chaotic motion be responsible for the formation of spiral arms?

Abstract

We investigate a number of iso‐energetic N‐body models of rotating galaxies, i.e. models that have the same total mass and the same total binding energy, but different amount of total angular momentum. These models are also in the same scalar virial equilibrium. We show that density waves grow so that a rotating strong bar is formed embedded in a rotating thick disk. The surface density of the disk has an exponential radial profile. On the disk, density waves form spiral arms. The longevity of the spiral arms increases with increasing angular momentum of the model. Separating the particles moving in regular orbits from the particles moving in chaotic orbits, we find that chaotic orbits have a considerably different spatial distribution than the regular orbits. Regular orbits form mainly a part of the central bar that works as a ballast for the stability of the bar. The disk and the spiral arms are almost completely formed by chaotic orbits. We conclude therefore that chaotic orbits can form disks with exponential surface density profiles, endowed with spiral arms. This new result reveals that chaotic orbits could be important in understanding the structure and evolution of exponential disks with spiral arms, at least in the case of strong bars.