The thought of two galaxies colliding seems fairly unlikely. After all, there’s so much space in the universe. But while the Milky Way is fairly isolated in this section of the cosmos, elsewhere, things can get pretty crowded. Galaxies pass by in much closer proximity, which frequently leads to interactions. The gravitational pull of these interacting galaxies might be mild, or it can lead to collisions between galaxies and an eventual galaxy merger.
Less extreme collisions are known as interactions. These typically occur between a larger galaxy and its satellite galaxies. The satellite galaxy might attract the outreaches of the primary galaxies' spiral arms, or their paths might interact and merge.
In some cases, two galaxies passing by will have enough momentum to keep on going. They won’t fall into one another, but carry on traveling after the interaction.
However, sometimes the interaction will have a more long-lasting effect. If galaxies collide and they lack momentum to move past, they can fall back into each other. This will cause the two galaxies to merge. This most often creates an irregular galaxy, but can also result in an elliptical galaxy.
When a much larger galaxy collides with a smaller galaxy, this is galactic cannibalism. The larger galaxy will look almost undisturbed, while the smaller galaxy will be torn apart. The remains of the smaller galaxy will be distributed across the bigger cannibal galaxy.
Within the galaxies, the changes can be intense. The gravitational potential, which is the shape of the galaxy, will quickly alter, knocking stars into new orbits far removed from their previous cycle. As a result, these merged galaxies are often full of stars moving in strange, even random, patterns. In elliptical galaxies, which are often created by mergers, complex orbit networks dominate.
Interacting galaxies can also result in galaxy harassment. Galaxy harassment occurs between a bright galaxy and a low-luminosity galaxy. When these two galaxies interact regularly, caused by high speeds and a high galactic density, the shape of both galaxies can distort. The end result can transform spiral galaxies into dwarf spheroidals and dwarf ellipticals.