Active galactic nucleus

An active galactic nucleus, or AGN, is a compact region at the center of a galaxy. This region emits a massive amount of energy across the electromagnetic spectrum. This energy is not produced by stars, but by other processes. We can observe this radiation in many forms, including radio, microwave, infrared, optical, ultraviolet, X-ray, and gamma rays. Because they are so bright, AGNs are the most luminous persistent sources of radiation in the universe. Scientists use them to find very distant objects in space. Studying how they change over cosmic time also helps us understand how the whole cosmos evolves.

The energy of an AGN comes from a process called accretion. This happens when a supermassive black hole pulls in matter from its surroundings. These black holes are enormous, with masses between 10^6 and 10^10 times that of our Sun. As gas and dust fall toward the black hole, they form an accretion disc. This is a flat, spinning ring of material. As the material moves inward, it heats up due to friction and energy changes. This hot disc peaks in the optical and ultraviolet wavebands. A corona of even hotter material often forms above the disc. This corona can scatter photons to create high-energy X-rays.

Some AGNs also produce relativistic jets. These are twin, highly collimated streams of matter. They shoot out in opposite directions from near the accretion disc. The direction of these jets depends on the spin of the black hole or the axis of the disc. These jets move at very high speeds. They emit radiation across all wavebands, from radio to gamma rays. One famous example is the jet in the galaxy M87. This jet is about 5,000 light-years long.

Astronomers have identified many different types of AGN. The most powerful ones are called quasars. In early photos, quasars looked like single stars, so they were called quasi-stellar radio sources. Another type is a blazar. A blazar is an AGN with a jet pointed directly toward Earth. This orientation causes the radiation to be enhanced by relativistic beaming. There are also Seyfert galaxies, which are often radio-quiet. These are galaxies where the bright nucleus can still be seen clearly. Scientists also classify them by whether they have broad emission lines in their light spectra.

The discovery of AGNs happened in stages over many years. In 1909, Edward Fath detected special emission lines in certain nuclei. In 1918, Heber Curtis discovered a jet in the galaxy Messier 87. In 1943, Carl Seyfert described galaxies with unusually bright nuclei. These became known as Seyfert galaxies in his honor. In the early 1950s, Viktor Ambartsumian proposed that these nuclei were active. He argued that explosions in the nuclei expelled large amounts of mass. This required the presence of bodies with huge, unknown masses.

In 1963, Maarten Schmidt made a major breakthrough. He measured the redshift of the quasar 3C 273. He found a redshift of 0.158, which proved the object was extremely far away. This meant the quasar was 100 times more powerful than other known radio galaxies. In 1964, Edwin Salpeter and Yakov Zeldovich suggested that accretion onto a black hole powered these objects. Later, in 1969, Donald Lynden-Bell proposed that nearby galaxies contain black holes that are relics of dead quasars. This helped explain why some galaxies have active nuclei while others do not.

Not all accretion looks the same. Some AGNs are "radiatively inefficient." This means they do not release energy as light very effectively. One theory for this is called Advection Dominated Accretion Flow, or ADAF. In this state, the matter does not form a thin, bright disc. Instead, the energy stays trapped in the moving matter rather than being radiated away. This helps explain why some massive black holes in elliptical galaxies do not show strong AGN radiation. These objects are much dimmer than standard AGNs.

Understanding AGNs connects to many parts of astronomy. They help us study the growth of black holes and how they affect their host galaxies. We know that most massive galaxies likely have a supermassive black hole at their center. The mass of these black holes relates to the brightness of the galaxy's bulge. Even our own Milky Way has a supermassive black hole. It is not active today, but it may have been active about 8 billion years ago. By studying these bright engines, we learn how the universe grew and changed.