Active Galactic Nucleus (AGN)

Definition of Active Galactic Nucleus (AGN)

  • An Active Galactic Nucleus (AGN) refers to the compact region at the centre of a galaxy.
  • This central area can outshine the entire galaxy due to its significant energy output.
  • A galaxy hosting an AGN is often referred to as an active galaxy.
  • The high-energy radiation from AGN is thought to be the result of accretion of mass onto the supermassive black hole at the centre of the host galaxy.

Types of Active Galactic Nuclei

  • There are several types of AGN, including Quasars, Seyfert galaxies, Blazars, and Radio galaxies.
  • Quasars are the most energetic and distant members of the AGN family. They can emit energy at a rate greater than an average galaxy, up to a thousand times more.
  • Seyfert galaxies are spiral galaxies with small, bright nuclei that also produce non-stellar emission lines.
  • Blazars are highly variable sources of electromagnetic radiation, notable for their rapid changes in brightness and spectra.
  • Radio galaxies exhibit powerful radio emission from their nuclei, often associated with massive jets extending to large distances.

Features of Active Galactic Nuclei

  • AGNs are characterised by their variable brightness, which can change over periods from months to minutes.
  • They emit energy across the entire electromagnetic spectrum, from radio waves to gamma rays.
  • The energy emitted can outshine the entire host galaxy, which implies an extremely efficient source of power.
  • This intense power is believed to originate from accretion of matter onto a supermassive black hole at the centre of the host galaxy.

Observations of Active Galactic Nuclei

  • AGNs are observed with both ground-based telescopes and orbiting observatories, which can detect X-rays and gamma rays that do not penetrate the Earth’s atmosphere.
  • Quasars, the brightest type of AGN, can be seen across great cosmic distances, providing information about the early universe.
  • AGNs provide vital clues about the co-evolution of galaxies and their central supermassive black holes.
  • Understanding AGNs is crucial in gaining insights into the distribution of galaxies in the universe and the large-scale cosmic web.