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N.F. Comins, T. Zeltwanger (University of Maine), R.V.E. Lovelace (Cornell University)
Observations of lopsided spiral galaxies motivate us to study possible causes and several effects of such asymmetry using an N-body simulation. Our test galaxy particles are free to move while embedded in a fixed halo. The causal agent we explore is the rapid passage of a companion galaxy. We find that the center of mass of the disk becomes offset relative to the halo mass; a transient one-armed spiral forms and persists for several rotation periods. After the passage, the test disk's orbit around the halo center of mass spirals back to the halos center.
We also studied cases where the disk's center of mass is initially offset relative to the halo's center by an amount typically found after the fast passage of a companion galaxy. We used five different halo mass distributions in these runs. We find that as the disk's center of mass spirals inward, it creates a variety of observed or observable phenomena including one-armed spirals, massive clumps of particles, and counter-rotating waves.
The observed one-armed spiral waves are in most cases trailing in accord with the linear theory of the eccentric motion of galactic disks embedded in a dark matter halo. Whether or not the long-term evolution of the disk exhibits a bar depends on the initial conditions.
Two of us (N.F.C. and T.Z.) wish to thank Sun Microsystems, Inc., and Electronic Data Systems for a grant of computers on which some of this work was done. The work of R.V.E.L. was supported in part by NSF grant AST 93-20068.