A galaxy

What Are Active Galaxies?

The Milky Way is a fairly calm galaxy. Other than the occasional supernova, our home galaxy is pretty inactive. There are some galaxies, however, that are far from inactive. These types of galaxies are called active galaxies, and it is believed that every galaxy was active at one point during their existence. What is an active galaxy and what makes them active?

How Galaxies Become Active

Centaurus A
Centaurus A is the closest active galaxy to the Milky Way. Image credit: NASA/ESA

An active galaxy is any galaxy whose energy output is extraordinarily 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 will emit in 30,000 years, which goes to show just how energetic they are. Most of this energy is found in the core of active galaxies. Interestingly, the cores of most galaxies in the universe are home to supermassive black holes, and so the formation and evolution of active galaxies is obviously tied to these massive objects. Furthermore, supermassive black holes tend to emit vast amounts of X-ray radiation, and active galaxies are known to be rich in X-rays. 

Within the core of an active galaxy, a supermassive black hole is emitting vast amounts of high energy radiation, yet why is this the case? A supermassive black hole can become active if there is a gigantic influx of material. As material falls into a black hole, it forms a gigantic disk called an accretion disk. If enough matter falls into the black hole at once, a tremendous amount of friction is created. This friction emits vast amounts of high energy radiation that flows outwards, causing a galaxy to become active. In order for there to be such a large influx of material, a galaxy must generally undergo a collision with another galaxy. When two galaxies collide, they exchange material, some of which falls into 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 a quasar, which are the brightest, most energetic events in the cosmos.

The Early Universe

Cigar Galaxy
This image shows the active galaxy known as the Cigar Galaxy. Image credit: NASA/ESA

Interestingly, active galaxies are far 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 are many billions of light years away, and there’s a reason for this. The early universe was smaller than the universe we now inhabit, and so galaxies were much closer together. Galactic collisions were thus far more common in the early universe, and so galaxies were far more likely to become active. The Milky Way is estimated to have first formed around 12-billion years ago, and it likely underwent an active phase as well. 

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