Evidence for Collisions
How can we tell when two galaxies that lie close to each other on the celestial sphere are interacting
with each other? First, they must lie at the same distance, not just in the same direction. Second, if they
are interacting there is usually observational evidence: tidal distortion, unusual star formation activity,
jets of matter, trailing bridges of matter, or indications of nonthermal emission of radiation are common
examples.