2008ApJ...683...94O

We investigate the physical properties of tidal structures in a disk galaxy created by gravitational interactions with a companion using numerical N-body simulations. We consider a simple galaxy model consisting of a rigid halo/bulge and an infinitesimally thin stellar disk with Toomre parameter Q~2. A perturbing companion is treated as a point mass moving on a prograde parabolic orbit, with varying mass and pericenter distance. Tidal interactions produce well-defined spiral arms and extended tidal features, such as bridge and tail, that are all transient, but distinct in nature. In the extended disks, a strong tidal force is able to lock the perturbed epicycle phases of the near-side particles to the perturber, shaping them into a tidal bridge that corotates with the perturber. A tidal tail develops on the opposite side as strongly perturbed, near-side particles overtake mildly perturbed, far-side particles. The tail is essentially a narrow material arm with a roughly logarithmic shape, dissolving with time because of large velocity dispersions. Inside the disks where the tidal force is relatively weak, on the other hand, a two-armed logarithmic spiral pattern emerges due to the kinematic alignment of perturbed particle orbits. While self-gravity makes the spiral arms a bit stronger, the arms never become fully self-gravitating, wind up progressively with time, and decay almost exponentially after the peak in a timescale of ∼1 Gyr. The arm pattern speed varying with both radius and time converges to Ω-κ/2 at late time, suggesting that the pattern speed of tidally driven arms may depend on radius in real galaxies. Here Ω and κ denote the angular and epicycle frequencies, respectively. We present the parametric dependences of various properties of tidal features on the tidal strength and discuss our findings as applied to tidal spiral arms in grand-design spiral galaxies.