Close encounter between galaxies – II. Tidal deformation of a disc galaxy stabilized by massive halo

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Abstract
A series of disc galaxy models are constructed and their deformation in the tidal interaction is investigated by N-body simulations. Each galaxy model is composed of a self-gravitating disc made up of many particles and a rigid halo component added to stabilize the disc. Each model is characterized by two parameters; the mass fraction of the disc and the mass concentration of the halo toward the centre. These parameters are widely varied to investigate the dependence on internal structure whereas the relative motions of two galaxies are resticted to parabolic prograde planar encounters. It is found that the self-gravitating discs, when perturbed by the tidal force of another galaxy, develop prominent spiral structures not only in the outer region but also in the inner region. This is a remarkable contrast to the case of the massless discs constructed by test particles, in which only the outer part exhibits a spiral structure. Moreover, the spiral structure generally has a bar-like shape in the inner region. The bar formation process depends upon the model parameters as follows. (i) As the disc-to-halo mass ratio increases, the bar forms more rapidly and its rotation rate is larger. The size of the bar becomes a little larger. (ii) As the halo becomes more strongly concentrated to the centre, the bar forms more rapidly and rotates faster. It becomes shorter in length. (iii) As the perturber becomes more massive, the bar forms more rapidly and becomes much longer, but the rotation rate decreases a little. Based on these results, we predict an overabundance of bars in interacting galaxies.

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