The backbones of stellar structures in barred-spiral models-the concerted action of various dynamical mechanisms on galactic discs

Abstract

We investigate the dynamics of a barred-spiral model, rotating with a single pattern speed, which is characterized by a corotation-to-bar-radius ratio (Rc/Rb) about 2.9. The response morphology of the model consists of an inner barred-spiral structure, surrounded by an ovalshaped disc and a fainter set of arms at larger radii. The oval-shaped disc and the barred-spiral structure included in it are located inside corotation, while the outer spiral arms extend beyond it. The system harbours two main different dynamical mechanisms, which shape its morphology. The bar and the spiral arms inside corotation are structured to a large extent by regular orbits, while the spiral arms beyond corotation are built by chaotic orbits. Chaotic orbits play a role inside corotation also, specifically in building weak extensions of the inner spirals as well as in the central part of the bar. The oval-shaped disc is also shaped by chaotic orbits. For the outer spirals, we find that the vast majority of the chaotic orbits, which reinforce the spirals at least for a time interval of eight pattern rotations, includes in its morphology the imprints of '4:1-resonance-like' orbits, in agreement with previous studies, as well as of 'long-period-banana-like' orbits. Both of them belong to orbits of the 'hot orbital population' that visit both areas, inside and outside corotation. This orbital population plays the key role for supporting structures out of chaos. In the case we study, order and chaos cooperate in building a galactic morphology that is encountered among grand design spiral galaxies (NGC 1566 and NGC 5248). The fact that in the model are implicated on one hand the 'precessing ellipses flow' supporting the spiral arms of normal spirals and on the other hand the 'chaotic spirals' found in barred-spiral systems, indicates that it is a model bridging two different orbital stellar dynamics. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.