An active black hole, also known as an active galactic nucleus (AGN), is a supermassive black hole at the center of a galaxy that is actively accreting matter from its surroundings. These black holes are characterized by the emission of significant amounts of electromagnetic radiation across multiple wavelengths, from radio waves to X-rays and even gamma rays.
The accretion process occurs when surrounding gas, dust, and stars are drawn toward the black hole by its immense gravitational pull. As this material falls into the black hole's vicinity, it forms an accretion disk—a swirling disk of hot, glowing matter—around the black hole. The energy released during this process produces intense radiation that can outshine an entire galaxy.
Active black holes exhibit a range of phenomena, including:
Quasars: These are the most luminous AGNs, appearing as extremely bright and distant objects in the universe. Quasars are powered by the accretion of massive amounts of matter onto the central black hole.
Seyfert Galaxies: These are galaxies that host active black holes, but with lower luminosities compared to quasars. Seyfert galaxies typically show emission lines in their spectra, indicating the presence of ionized gas around the black hole.
Radio Galaxies: These are AGNs that emit strong radio waves. They often have enormous jets of charged particles that extend far beyond the galaxy itself.
Blazars: Blazars are a specific type of active black hole with relativistic jets pointing directly toward Earth. This alignment results in highly variable and extremely energetic emissions across the electromagnetic spectrum.
Active black holes are believed to play a crucial role in galaxy evolution and the formation of structures in the universe. The processes occurring in their vicinity, including the powerful jets they generate, can influence star formation, galaxy mergers, and the distribution of matter in their host galaxies. Studying active black holes helps us understand the physics of extreme gravitational environments and the interactions between black holes and their surroundings.