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Dynamical Properties of Tidally Induced Galactic Bars

We simulated a series of fully three-dimensional N-body models of tidal encounters between a disk galaxy and a perturbing galaxy to investigate the dynamical properties of tidally induced galactic bars, especially in connection with the resonance structure, which we have little knowledge about. We also calculated another set of N-body models on isolated galaxies with bar-unstable disks to make a comparison between galactic bars of different origins. To reveal the resonances in a highly nonaxisymmetric potential for which the ordinary method of deriving resonances using the Ω ± κ/2 curves gives unreliable results, we employed the analysis based on the families of periodic orbits in a barred potential. It is found from our simulations that the tidally induced bars sometimes rotate quite slowly and have inner Lindblad resonances (ILR) near the bar ends, whereas the spontaneously formed bars have no ILR and end near corotation because of their fast rotation. Since the difference in resonance structures affects the kinematics of the interstellar gas, these peculiar bars terminated by ILRs may give us a new way of creating the vast morphological and kinematical variety observed in the real barred galaxies, which may not be explained solely by the spontaneous bars. Slow rotation of the tidal bar is caused by two major factors. First, the small mass fraction of the disk in which a tidal bar is created leads to a small pattern speed. Second, the angular momentum transfer from the inner disk to the perturber at the time when the perturber passes the pericenter serves to reduce the pattern speed. The former effect is observed most clearly in the light disk models in which only 10% of the total mass of the galaxy is ascribed to the disk component. On the other hand, massive disk models in which the disk is stabilized by large random motions in disk stars are quite sensitive to the second process. This complicated behavior of tidal bars can be understood naturally by recognizing two regimes of tidal bar formation. When the tidal perturbation is relatively weak, it works only as a trigger of bar formation, and the bar properties are determined largely by the internal structure of the target galaxy. On the other hand, a sufficiently strong tidal perturbation washes out the intrinsic property of the target galaxy and imposes on the bar a common characteristic determined by the parameters of the tidal encounter.