1999A&A...348..737R

We have studied the coexistence of several modes with different pattern speeds in barred galaxies, by a simulation survey exploring a wide range of initial conditions. The high resolution two-dimensional experiments cover the dynamical evolution for about one Hubble time. A remarkable feature of these simulations is that in many cases the spiral structure is clearly visible in the stellar component for several gigayears and weakens so slowly that it can take more than 10 gigayears to become indiscernable even in the direct density plots. We confirm Sellwood and Sparke's (1988MNRAS.231p..25S) results that the pattern speed of the spiral arms may differ from that of the bar. However, we find several different variations. There are systems where the bar and the spiral structure are clearly corotating, whereas in others they have different pattern speeds but are probably connected by a non-linear mode coupling as was suggested by Tagger et al. (1987ApJ...318L..43T). We have also found models with separate pattern speeds, but without evident mode coupling. Several simultaneous spiral modes can also coexist in the disk, even overlapping in radius. Sometimes the systems have separate inner and outer spirals, the inner corotating with the bar and the outer having a lower pattern speed. We conclude that similar variation can exist in real galaxies. Some morphological features, like distinct spiral structures with large size differences, can be best explained by separate coexisting patterns. However, there are cases where most of the disk is dominated by one mode. For example, galaxies with well developed outer rings are probably genuine examples of corotating features. It is also possible that in the presence of two or more modes, the appearance of the spiral structure changes considerably in a time scale of about one gigayear while the Hubble type of the galaxy stays the same.