Active Galactic Nuclei (AGN) – Definition & Detailed Explanation – Astrophysics Glossary

I. What is an Active Galactic Nucleus (AGN)?

An Active Galactic Nucleus (AGN) is a compact region at the center of a galaxy that emits large amounts of energy across the electromagnetic spectrum. AGNs are powered by accretion of material onto a supermassive black hole located at the center of the galaxy. This accretion process releases a tremendous amount of energy, making AGNs some of the most luminous objects in the universe.

AGNs are characterized by their high levels of energy emission, which can be observed in various wavelengths, including radio, infrared, optical, ultraviolet, and X-ray. The energy emitted by AGNs can outshine the entire galaxy in which they reside, making them key objects of study for astronomers.

II. How are AGNs classified?

AGNs are classified based on their observed properties, including their emission lines, luminosity, and morphology. The most common classification scheme for AGNs is based on the presence or absence of broad emission lines in their spectra. AGNs with broad emission lines are known as Type 1 AGNs, while those without broad emission lines are classified as Type 2 AGNs.

Another classification scheme for AGNs is based on their luminosity, with AGNs being categorized as either low-luminosity AGNs or high-luminosity AGNs. Low-luminosity AGNs are typically found in less massive galaxies, while high-luminosity AGNs are associated with more massive galaxies.

III. What are the main components of an AGN?

The main components of an AGN include the supermassive black hole at the center, an accretion disk of gas and dust surrounding the black hole, and a region of ionized gas known as the broad-line region. The accretion disk is responsible for funneling material onto the black hole, where it releases energy in the form of radiation and jets of high-energy particles.

The broad-line region is located close to the accretion disk and emits broad emission lines due to the intense radiation from the accretion disk. Surrounding the broad-line region is the narrow-line region, which emits narrow emission lines and is located farther away from the accretion disk.

IV. How do AGNs produce such high levels of energy?

AGNs produce high levels of energy through the process of accretion onto the supermassive black hole at their center. As material from the surrounding galaxy falls onto the black hole, it forms an accretion disk that heats up and emits radiation across the electromagnetic spectrum.

The energy released by the accretion disk can be in the form of X-rays, ultraviolet radiation, and even gamma rays. In addition to radiation, AGNs can also produce powerful jets of high-energy particles that can extend for thousands of light-years away from the central black hole.

V. What are the different types of AGNs?

There are several different types of AGNs, each characterized by their unique properties and emission signatures. Some of the most common types of AGNs include Seyfert galaxies, quasars, and blazars.

Seyfert galaxies are low-luminosity AGNs that exhibit narrow emission lines in their spectra. Quasars, on the other hand, are high-luminosity AGNs that emit large amounts of energy across the electromagnetic spectrum. Blazars are a type of AGN with relativistic jets that are pointed directly towards Earth, making them particularly bright and variable sources of energy.

VI. How do astronomers study AGNs?

Astronomers study AGNs using a variety of observational techniques, including spectroscopy, imaging, and monitoring of their variability over time. Spectroscopy is used to analyze the emission lines in the spectra of AGNs, providing valuable information about their physical properties and the dynamics of the gas surrounding the central black hole.

Imaging techniques allow astronomers to study the morphology of AGNs and their surrounding environment, providing insights into the structure and evolution of these energetic objects. Monitoring the variability of AGNs over time can reveal information about the processes occurring near the central black hole, such as accretion events and the launching of jets.

Overall, the study of AGNs is crucial for understanding the role of supermassive black holes in the evolution of galaxies and the universe as a whole. By unraveling the mysteries of these powerful objects, astronomers can gain valuable insights into the processes that drive the growth and evolution of galaxies across cosmic time.