Active Galactic Nuclei

Definition

Active galactic nuclei (AGN) refer to the extremely luminous and energetic centers of some galaxies, powered by supermassive black holes that are actively accreting matter. These galactic nuclei emit radiation across the entire electromagnetic spectrum, from radio waves to high-energy gamma rays, making them some of the most energetic phenomena in the universe.

5 Must Know Facts For Your Next Test

1. Active galactic nuclei are believed to be powered by the accretion of matter onto supermassive black holes at the centers of galaxies.
2. The intense radiation emitted by AGN can outshine the entire host galaxy, making them visible across vast cosmic distances.
3. AGN are classified into different types based on their observed properties, such as the presence of emission lines, the orientation of the accretion disk, and the level of activity.
4. The energy output of AGN can have a significant impact on the evolution of their host galaxies, through processes like jet-driven feedback and radiative heating.
5. Studying the properties and evolution of AGN provides valuable insights into the formation and growth of supermassive black holes, as well as the co-evolution of galaxies and their central black holes.

Review Questions

• Explain how active galactic nuclei are related to the concept of cosmic rays discussed in Section 20.4.
  • Active galactic nuclei are thought to be one of the primary sources of the most energetic cosmic rays observed in the universe. The powerful jets and outflows associated with AGN can accelerate charged particles to extremely high energies, producing the high-energy cosmic rays that are detected on Earth. The study of cosmic rays can therefore provide insights into the physical processes occurring within the central regions of active galaxies, where the supermassive black holes and accretion disks reside.
• Describe how the evidence for supermassive black holes discussed in Section 24.6 relates to the phenomenon of active galactic nuclei.
  • The presence of supermassive black holes at the centers of galaxies is a key component of the active galactic nuclei phenomenon. The intense gravitational fields and high-energy processes associated with these black holes are believed to power the extreme luminosity and radiation output observed in AGN. Observations of the dynamics and kinematics of stars and gas within the central regions of galaxies have provided strong evidence for the existence of these supermassive black holes, which are now understood to be the central engines driving the most energetic phenomena in the universe, including active galactic nuclei.
• Analyze how the different types of galaxies discussed in Section 26.2, as well as their properties described in Section 26.3, may be related to the occurrence and characteristics of active galactic nuclei.
  • The type of galaxy and its overall properties can have a significant influence on the presence and characteristics of active galactic nuclei. Elliptical galaxies, which are typically more massive and have higher central densities, are more likely to host supermassive black holes that can power AGN. In contrast, spiral galaxies like the Milky Way, which have lower central densities, are less likely to exhibit strong AGN activity. The properties of a galaxy, such as its gas content, star formation rate, and the dynamics of its central region, can also affect the accretion rate and power output of the supermassive black hole, and thus the observed characteristics of the active galactic nucleus. Understanding these connections between galaxy types, properties, and AGN is crucial for studying the co-evolution of galaxies and their central black holes, as discussed in Section 28.5.

Related terms

Supermassive Black Hole: A black hole with a mass millions to billions of times greater than the Sun, typically found at the center of galaxies.

Accretion Disk: A rotating disk of gas and dust that surrounds and feeds material into a central object, such as a black hole or a young star.

Quasar: The extremely luminous and energetic core of an active galaxy, powered by the accretion of matter onto a supermassive black hole.