Order and chaos in spiral galaxies seen through their morphology

Abstract

It is clear from dynamical considerations and N-body models that both order and chaos are important for spiral galaxies. To study the relative importance of order and chaos in real galaxies, one needs accurate kinematic information for the stellar population. Unfortunately, such data are still very difficult to obtain outside the very central parts of spiral galaxies due to the relative low surface brightness of their disks. As a second option, one can consider morphological features in the disks (e.g. bars, and spiral arms) and interpret them as indicators of the underlying dynamics. The presence of large-scale, well-define structures (such as bars and grand-design spirals) in disks of many spiral galaxies suggests that collective phenomena like density waves supported by regular orbits are important. The alignment of both young objects and dust lanes along spiral arms indicates a regular gas flow which would be shared by newly formed stars. At the end of bars, where one expects their co-rotation, a more diffuse region is often seen, possibly caused by chaotic stars. A few percent of barred galaxies, mainly early-type spirals, show broad, tight spirals with a morphology very close to that seen in N-body models where the arms are dominated by stars on chaotic orbits.