Active galactic nucleus

An active galactic nucleus (AGN) is a very bright area at the center of a galaxy. This region gives off huge amounts of energy across many types of light, including radio, microwave, infrared, optical, ultra-violet, X-ray, and gamma ray waves. This energy does not come from stars, but is thought to be produced as matter falls into a giant supermassive black hole at the galaxy’s center.

The Milky Way, the galaxy we live in, has a supermassive black hole too, but it is not active right now. Scientists believe it was active about 8 billion years ago.

AGNs are some of the brightest objects in the universe. Because they are so bright, astronomers can see them even when they are very far away. Studying AGNs helps scientists learn about how the universe has changed over time and test ideas about how the universe works, known as models of the cosmos. Different types of AGNs look different depending on the size of the black hole, how fast material is falling into it, how the material is arranged, and whether there are powerful streams of energy, called jets, shooting out from the center. The brightest and most powerful AGNs are called quasars, and a special kind called a blazar has a jet pointing toward Earth, making it appear even brighter.

History

During the first half of the 20th century, scientists began noticing special signals coming from the centers of galaxies. These signals were different from the light of stars and puzzled astronomers. Important early discoveries included seeing unusual light from galaxies like NGC 1068 and Messier 81, and spotting a jet of energy from Messier 87.

The development of radio telescopes helped scientists learn more. They found powerful radio sources in galaxies such as Messier 87 and Centaurus A. One famous radio source, Cygnus A, was found to be a distant galaxy. These discoveries led to the idea of active galactic nuclei, which are very bright centers of galaxies. Today, studying these active centers helps scientists understand how galaxies grow and change.

Models

Since the late 1960s, scientists have believed that an active galactic nucleus (AGN) gets its energy from material falling into a huge black hole at its center. These black holes can be millions to billions of times the mass of our Sun, and they can shine very brightly for a long time.

In many models, material forms a flat, spinning disk around the black hole. This disk heats up and glows especially in visible and ultraviolet light. Sometimes, very fast streams of particles, called jets, shoot out from near the black hole in opposite directions. These jets can be seen across many types of light, from radio waves to gamma rays. There are also some types of AGN that don't shine as brightly, which helps explain why some big galaxies don't look as bright as we might expect.

Particle acceleration

Active galactic nuclei (AGN) are thought to be sources of high and ultra-high energy cosmic rays. These tiny particles travel through space at nearly the speed of light. One possible way these particles gain their speed is through a process called the centrifugal mechanism of acceleration.

Observational characteristics

Active galactic nuclei (AGNs) are very bright, shining across huge distances, which we call high red shifts. The areas that produce this light are also very small, only about a few milli-parsecs across, meaning they pack a lot of energy into a tiny space. AGNs can emit light in many different forms, from radio waves to gamma rays, showing their energy across the whole electromagnetic spectrum.

Main article: luminosity functions
Further information: electromagnetic spectrum

Types

Scientists sort active galactic nuclei (AGNs) into many groups based on how they look in different kinds of light, like radio waves or visible light. Sometimes an AGN can be in one group when looked at with one type of light and in another group when looked at with a different type of light. This is because scientists are still learning about AGNs.

One way to sort AGNs is by how bright they are in radio waves compared to visible light. If an AGN is very bright in radio waves, it is called "radio-loud." If it is not very bright in radio waves, it is called "radio-quiet." Another way to sort AGNs is by whether they show certain bright lines in their visible light, called broad emission lines. AGNs that show these lines are called type-1, and those that do not are called type-2.

Radio-quiet AGN

When an AGN is in a galaxy that we can see clearly, it is called a Seyfert galaxy. These galaxies are sorted into type-1 or type-2 based on whether they show broad emission lines.

Radio-loud AGN

Radio-loud AGNs come in several types. Some act like radio-quiet AGNs but also send out strong radio waves from a jet. Others, called blazars, change quickly in brightness and can be seen in radio, visible, and X-ray light. Radio galaxies send out strong radio waves and can be different in other ways, depending on whether they show certain bright lines in their light.

Unified models

Unified models suggest that different types of active galactic nuclei (AGN) might actually be the same kind of object, but we see them differently depending on our viewpoint. One idea is that the way we see these objects changes based on the angle we look at them from Earth, especially because of a dusty layer surrounding a disk of material.

Scientists also think other factors, like how fast material is falling into the center of the galaxy or how strong the energy jet is, can make AGNs look different. Researchers are still studying AGNs to learn more about what they are.

Effects on planets

Supermassive black holes at the centers of galaxies go through periods of high activity, which can affect planets and their ability to support life. Planets in smaller galaxies might be more affected than those in larger ones. For planets with a lot of oxygen, this activity could help create a thicker ozone layer, making them more habitable.

The black hole in our galaxy, Sagittarius A*, had a period of high activity 8 billion years ago. This could have stripped away atmospheres from planets nearby and harmed any life that might have existed there. The energy from such activity can also heat up planetary atmospheres, making them unable to hold onto their air.

Examples

An active galactic nucleus (AGN) is found at the center of some galaxies and shines very brightly. This brightness comes from something other than stars, and it can be seen across many types of light, from radio waves to gamma rays.

Some famous examples of galaxies with AGNs include:

• Quasars: These are very bright AGNs. Examples are 3C 273, 3C 48, and TON 618, which has one of the biggest black holes known.
• Radio Galaxies: These galaxies have powerful jets shooting out from their centers. Messier 87 and Centaurus A are well-known examples.
• Seyfert Galaxies: These have AGNs that shine brightly in certain kinds of light. Messier 77 and NGC 4151 are classic examples.
• Blazars: These are AGNs with jets pointed toward Earth, making them very bright and changeable. BL Lacertae is the first one discovered.