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. It 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

Quasar 3C 273 observed by the Hubble Space Telescope. The relativistic jet of 3C 273 appears to the left of the bright quasar, and the four straight lines pointing outward from the central source are diffraction spikes caused by the telescope optics.

In the early 1900s, scientists noticed strange signals from the centers of galaxies. These signals were different from starlight and puzzled astronomers. They saw unusual light from galaxies like NGC 1068 and Messier 81, and spotted a jet of energy from Messier 87.

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 a distant galaxy. These discoveries led to the idea of active galactic nuclei, which are very bright centers of galaxies. Studying these bright centers helps scientists learn how galaxies grow and change.

Models

Image taken by the Hubble Space Telescope of a 5000-light-year-long jet ejected from the active galaxy M87. The blue synchrotron radiation contrasts with the yellow starlight from the host galaxy.

Since the late 1960s, scientists have thought 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) may produce high-energy cosmic rays. These tiny particles move through space very fast, almost at the speed of light. One way they might get this speed is through a process called the centrifugal mechanism.

Observational characteristics

Active galactic nuclei (AGNs) are very bright objects that shine across huge distances. They can be seen even though they are far away. The areas that produce this light are very small, only about a few milli-parsecs across. This means they have a lot of energy in a tiny space. AGNs can emit light in many different forms, from radio waves to gamma rays. This shows their energy across the whole electromagnetic spectrum.

Main article: luminosity functions
Further information: electromagnetic spectrum

Types

Scientists group active galactic nuclei (AGNs) in many ways. They look at how AGNs appear in different kinds of light, like radio waves or visible light. Sometimes an AGN can look different when viewed with different types of light. This is because scientists are still learning about AGNs.

One way to group 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 group 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.

Features of different types of galaxies
Galaxy type	Active nuclei	Emission lines		X-rays	Excess of		Strong radio	Jets	Variable	Radio loud
Galaxy type	Active nuclei	Narrow	Broad	X-rays	UV	Far-IR	Strong radio	Jets	Variable	Radio loud
Normal (non-AGN)	no	weak	no	weak	no	no	no	no	no	no
LINER	unknown	weak	weak	weak	no	no	no	no	no	no
Seyfert I	yes	yes	yes	some	some	yes	few	no	yes	no
Seyfert II	yes	yes	no	some	some	yes	few	no	yes	no
Quasar	yes	yes	yes	some	yes	yes	some	some	yes	some
Blazar	yes	no	some	yes	yes	no	yes	yes	yes	yes
BL Lac	yes	no	no/faint	yes	yes	no	yes	yes	yes	yes
OVV	yes	no	stronger than BL Lac	yes	yes	no	yes	yes	yes	yes
Radio galaxy	yes	some	some	some	some	yes	yes	yes	yes	yes

Unified models

Unified AGN models

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

Scientists 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 sometimes become very active. This activity can change planets and make it harder for them to support life. Smaller galaxies may be affected more than larger ones.

Planets with lots of oxygen might benefit from this activity. It could help create a thicker ozone layer, making the planets more friendly for life.

Our galaxy's black hole, Sagittarius A*, was very active 8 billion years ago. This might have removed atmospheres from nearby planets and harmed any life there. The energy from this activity can also heat up planetary atmospheres, making it hard for them to keep their air.

Examples

An active galactic nucleus (AGN) is found at the center of some galaxies and shines very brightly. This brightness does not come from stars. It can be seen in many kinds of light, from radio waves to gamma rays.

Some famous galaxies with AGNs include:

Quasars: These are very bright AGNs. Examples are 3C 273, 3C 48, and TON 618.
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.