Transcript: Galaxies in close proximity are called interacting galaxies. Interacting galaxies might be gravitationally bound to each other, or they may simply be passing on trajectories through the universe that bring them close together. Interacting galaxies are affected by each other’s gravity. Tidal forces can act to cause mass or gas to flow towards the centers, and gas can pass from one galaxy to the other. There’s good evidence that interacting galaxies show increased incidence of nuclear activity as manifested by Seyfert nuclei, radio emission, or x-ray emission. Thus interaction is a trigger for nuclear activity.