Sloshing in high-speed galaxy interactions

Observations of lopsided spiral galaxies motivated us to explore whether th e rapid passage of a companion galaxy could cause them. We examine whether the center of mass of the visible matter becomes displaced from the center of mass of the dark halo during the intruder's passage, thereby causing an asymmetric response and asymmetric structure. Two-dimensional N-body simula tions indicate that this can happen. We also explore some consequences of t his offset. These include the center of mass of the visible disk following a decaying orbit around the halo center of mass and the development of tran sient one-armed spirals that persist for up to six rotation periods. We the n study the results of a variety of initial conditions based on such offset s. We report on the results of several runs in which we initially offset a disk from its halo's center of mass by an amount typical of the above inter action. In some runs the halo is free to move, while in others it is held f ixed. We used three different mass distributions for the halo in these runs . We find that the disk's center of mass spiraled inward, creating a variet y of observed or observable phenomena including one-armed spirals, massive clumps of particles, and counterrotating waves. The systems settle into rel atively axisymmetric configurations. Whether or not the end states included a bar depended on a variety of initial conditions.