Active galactic nuclei

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits far more radiation than can be produced by stars alone. The luminosity is powered by accretion onto a supermassive black hole, typically with masses ranging from millions to billions of solar masses. The infalling matter heats up as it spirals through the accretion disk, producing radiation across the electromagnetic spectrum from radio waves to X-rays and gamma rays.

AGN are not uniform objects. They are classified by their observed properties — optical emission lines, radio loudness, X-ray luminosity — into categories including Seyfert galaxies, radio galaxies, quasars, and blazars. These classifications reflect different viewing angles, accretion rates, and black hole spin parameters rather than fundamentally different physical mechanisms. The unified model of AGN proposes that all AGN are essentially the same type of object viewed from different angles and at different scales.

The feedback from AGN — radiation pressure, winds, and relativistic jets — regulates star formation in the host galaxy and can heat or expel gas from the galactic halo. This AGN feedback is a critical component of modern cosmological simulations, explaining why massive galaxies stop forming stars and why the most massive black holes correlate so tightly with the properties of their host galaxies' stellar bulges.