An active black hole, also known as an active galactic nucleus (AGN), refers to a supermassive black hole that is actively accreting matter from its surrounding environment. These black holes are found at the centers of galaxies and exhibit powerful emission across a wide range of wavelengths, including radio waves, infrared, visible light, X-rays, and even gamma rays.
The accretion process occurs when surrounding gas, dust, and stars are drawn towards the black hole due to its immense gravitational pull. As this material falls into the black hole's gravitational well, it forms an accretion disk—a swirling disk of hot, glowing matter that emits intense radiation. The release of this energy is what makes active black holes visible and distinguishable from dormant or quiescent black holes.
Active black holes exhibit a variety of phenomena associated with their accretion process, such as:
Quasars: These are extremely bright, distant objects powered by active black holes. Quasars are some of the most luminous objects in the universe and are believed to be fueled by the accretion of large amounts of matter onto supermassive black holes.
Seyfert galaxies: These are spiral galaxies that harbor active black holes at their centers. Seyfert galaxies display characteristic emission lines in their spectra, indicating the presence of ionized gas in their nuclei.
Blazars: Blazars are a specific type of active black hole associated with a relativistic jet, a narrow beam of highly energetic particles emitted from the vicinity of the black hole. Blazars are observed when this jet is oriented nearly towards the observer, resulting in intense emissions across the electromagnetic spectrum.
The study of active black holes provides valuable insights into the physics of extreme gravity, accretion processes, and the interaction between black holes and their host galaxies. Scientists continue to investigate these phenomena to deepen our understanding of how galaxies and black holes evolve and shape the universe around them.