This paper presents self-consistent numerical results for a small sample of merging encounters between equal-mass disk galaxies. These calculations illustrate how self-gravitating disks respond to tidal perturbations and suggest an improved point of view on orbital decay in multicomponent systems. Preexisting spheroidal components merge rather gently, but the incomplete violent relaxation of the disks themselves is accompanied by a large drop in coarse-grained phase-space density. A detailed analysis of the orbital structure of these merger remnants shows how their shapes and kinematic properties are related to the initial disk spin vectors and other encounter parameters. Many of these remnants exhibit significant misalignment between their minor and rotation axes, a result which may constrain the number of elliptical galaxies formed by purely stellar-dynamical mergers.
1. Initial Conditions. Before their collision, two disk galaxies are rotated about the vertical axis. Each galaxy contains a thin disk (blue), a central bulge (yellow), and a dark halo (red). One disk lies in the orbit plane, and spins in the same direction that the galaxies circle each other. The other disk is inclined by an angle of 71 degrees.MPEG movie
2. Time Evolution. The galaxies approach each other, tidally interact as they pass each other, and fling out tidal bridges and tails. Subsequently they reach maximum separation, fall back together, and merge. The counter in the upper right shows time in units of 250 million years.MPEG movie
3. Rotation, t = 1.5. About 125 million years after first approach, the galaxies have developed extended bridges and tails. These are shown to advantage by rotating about the vertical axis as in this video.MPEG movie
4. Direct Disk. To show the reaction of the ``direct'' or in-plane disk, this video views this galaxy face-on while representing its partner as a dot.MPEG movie
5. Inclined Disk. Complementing the video above, this one views the inclined disk face-on while representing its partner as a dot.MPEG movie
6. Time Evolution. Following up the 2nd video above, this one shows the second approach, merger, and subsequent relaxation of the remnant.MPEG movie
7. Rotation, t = 3. Just after the merger, the remnant is rotated about the vertical axis to illustrate the shells, plumes, and tails typical of very young merger remnants.MPEG movie
8. Rotation, t = 6. About 750 million years after the merger, the remnant has substantially relaxed and appears more smooth and regular. Note the faint loops and tails which still surround it.
Computer simulations and video production were performed at the Pittsburgh Supercomputing Center (PSC). My thanks to Joel Welling and Anjana Kar at PSC for help in generating the videos.
Last modified: June 11, 1997