Two-Galaxy Collision

What you are seeing: two disk galaxies on a flyby trajectory, integrated with the shared 2D Barnes-Hut quadtree at $O(N \log N)$ . Prograde-prograde geometry, so the tidal field pulls long tails (the iconic Toomre antennae) out of each disk during pericentre

Figure 1. Two-galaxy fly-by under Barnes-Hut gravity, showing the development of Toomre antennae and eventual merger. Method: leapfrog kick-drift-kick, opening angle 0.7, Plummer softening.

disk stars / galaxy1200

separation4.0

approach speed0.70

opening θ0.70

WHAT TO TRY

Watch the two disks fly past: the Barnes-Hut gravity pulls long tidal tails and bridges out of each galaxy, the same structures seen in the Antennae and Mice galaxies.
Lower the approach speed or separation: a slower, closer pass does far more tidal damage, dragging out longer tails and eventually merging the cores.
Raise the star count or tighten the opening angle theta: the quadtree resolves the gravity more finely, sharpening the tidal features at the cost of more computation.