The Milky Way is a fairly quiet galaxy. Apart from the occasional supernovae, our home galaxy is fairly inactive. However, there are some galaxies that are far from idle. These types of galaxies are called active galaxies and each galaxy is thought to have been active at some point in its existence. What is an active galaxy and what makes them active?
How galaxies become active
An active galaxy is any galaxy whose energy output is extremely high. Active galaxies are among the most energetic objects in the universe. Even a small active galaxy will emit more energy in one second than the Sun emits in 30,000 years, showing how energetic they are. Most of this energy is contained in the nuclei of active galaxies. Interestingly, the cores of most galaxies in the universe are home to supermassive black holes, so the formation and evolution of active galaxies are obviously related to these massive objects. In addition, supermassive black holes tend to emit massive amounts of X-rays, and active galaxies are known to be rich in X-rays.
At the core of an active galaxy, a supermassive black hole is emitting massive amounts of high-energy radiation, but why is that? A supermassive black hole could become active if there is a giant influx of material. When material falls into a black hole, it forms a giant disk called an accretion disk. If enough matter enters the black hole at the same time, enormous friction is created. This friction releases a huge amount of high-energy radiation that flows outward, causing the galaxy to become active. For such a large influx of material to occur, a galaxy would normally have to undergo a collision with another galaxy. When two galaxies collide, they exchange matter, some of which ends up in the central black hole. With so much material falling into a supermassive black hole, the amount of energy generated is truly unfathomable. If there is a large enough influx of material, the energy generated can be so high that it produces quasars, which are the brightest and most energetic events in space.
Interestingly, active galaxies were much more common in the early universe than they are today. Most active galaxies are very far away, the nearest one being 11 million light-years away. However, most of them are many billions of light years away, and for good reason. The early universe was smaller than the universe we live in now, so the galaxies were much closer together. Thus, galactic collisions were much more common in the early universe, and therefore galaxies were much more likely to become active. The Milky Way is estimated to have first formed about 12 billion years ago and likely went through an active phase as well.