Slowly rotating bars-Morphologies introduced by bistability in barred-spiral galactic potentials

Abstract

We investigate the orbital dynamics of a \textit{barred-spiral} model when the system is rotating slowly and corotation is located beyond the end of the spiral arms. In the characteristic of the central family of periodic orbits we find a "bistable region". In the response model we observe a ring surrounding the bar and spiral arms starting tangential to the ring. This is a morphology resembling barred-spiral systems with inner rings. However, the dynamics associated with this structure in the case we study is different from that of a typical bar ending close to corotation. The ring of our model is round, or rather elongated perpendicular to the bar. It is associated with a folding (an "S" shaped feature) of the characteristic of the central family, which is typical in bistable bifurcations. Along the "S" part of the characteristic we have a change in the orientation of the periodic orbits from a x1-type to a x2-type morphology. The orbits populated in the response model change rather abruptly their orientation when reaching the lowest energy of the "S". The spirals of the model follow a standard "precessing ellipses flow" and the orbits building them have energies beyond the "S" region. The bar is structured mainly by sticky orbits from regions around the stability islands of the central family. This leads to the appearance of X-features in the bars \textit{on} the galactic plane. Such a bar morphology appears in the unsharp-masked images of some moderately inclined galaxies.