whatToomre and Toomre's 1972 computation, live: two galaxy centers -- softened point potentials on an exact two-body orbit -- and a few thousand massless disk stars riding the changing field. No gas, no dark matter, no star formation. Just the tides.

whyThe claim: one pass throws the tails. As the intruder swings by, the near side of each disk is pulled harder than the far side; stars launch into a bridge toward the companion and long curved tidal tails behind -- the Antennae, from arithmetic. And the deep half: it only works PROGRADE. Run the same pass against the disk's spin and it leaves the disk barely ruffled -- the HUD counts tail and bridge stars both ways.

howLeapfrog throughout; the centers' two-body energy drift is the honesty ledger. The census calls a star bridge (bound to the other galaxy), tail (bound, but flung past a disk radius), or escaped, and measures the tails' one-sidedness as the largest angular wedge. Drag anywhere: your arrow is the intruder's starting position and velocity, and the encounter replays. One integer seeds the disks.