Even though the distances between galaxies are typically large, many galaxies come close to their neighbors at some point in their long lives. When galaxies interact, collide, or merge, it is referred to as galaxy interactions. The Milky Way and Andromeda galaxies are separated by only 15 to 20 of their own diameters and are in orbits in our galaxy group that will one day bring them together. Colliding galaxies present an extraordinary situation in which the gravitational tugs and pulls of the systems on one another can trigger a myriad of effects ranging from simple distortions (such as tidal tails) and as dramatic as increased levels of star formation and the turning on of quasars.
Interacting Galaxy Pair Arp 87. Click here for original source URL
When galaxies travel through space, a near miss is more likely than a head-on collision, but galaxies do not actually have to collide for their properties to be changed. Gravity has a long reach, so a close encounter will produce a tidal interaction. Just as the Moon raises a tidal bulge in the Earth's oceans, so a nearby companion may distort the shape of a galaxy. Stars and gas will be pulled out on either side, and rotation will sling the material out into graceful, curving arcs. An example is the Magellanic stream, which is an arc of gas caused by an encounter between the Milky Way and the Magellanic clouds. The evidence of a tidal interaction does not last forever — within a few hundred million years (10% of the age of the universe) the flung-out stars will drift away or fall back into the bulk of the galaxy they came from. About a third to a half of all galaxies have had tidal interactions since they formed.
While gravitational interactions can distort galaxies, and cause their structures to fall apart and reform into something new. While this is happening, gas may frictionally interact and collide, but stars are so widely spaced that they almost never collide. This can be imagined as two handfuls of sand thrown at each other from across a room. The widely spaced clouds of flying grains of sand generally all pass by one another as they continue across the room. So too with stars in galaxies passing through one another.
While most galaxies only undergo glancing blows before gravitationally falling into a collisional heap, head-on collisions do on occasion happen. In a merger of two nearly equal-sized galaxies, gas clouds collide and are compressed. They become hotter and denser, making star formation much more likely. Star formation in turn heats the surrounding dust, causing intense far-infrared emission. Roughly 10% of all galaxies show distorted shapes from past collisions or close encounters. When two disk galaxies of similar size meet, transient streamers can be thrown off. If two galaxies of very different size meet, the larger galaxy will gobble up the smaller one. This is called galactic cannibalism. As a small galaxy approaches a much larger one, two things will happen. First, the gravity of the small galaxy will be too weak to retain its own outer stars, so they will be ripped away onto the larger galaxy. Second, as the core of the smaller galaxy ploughs through the outer regions of the larger galaxy, it will lose energy and decelerate, while the stars in the large galaxy heat up and accelerate. As a result, the core of the small galaxy spirals into the center of the larger galaxy.
The interaction and merger of galaxies is not just a game played on computers. In the past few years, astronomers have discovered several new dwarf companions of the Milky Way, many of which our galaxy is actively consuming. Using the Sloan Digital Sky Survey, a myriad of tidal tails have been found, marking the orbits of now disrupted systems. From small collisions, to large mergers, galaxies are forever evolving through interactions.