Astronomers theorize that collisions andmergers of small gas-rich galaxies result in the formation of massive galaxies. At the center, these objects contain a supermassiveblack hole (SMBHs) weighing more than one-million solar masses. When material accumulates onto a SMBH, the accretion disk surrounding it becomes extremely hot from the release of gravitational energy and it becomes very bright.

Astronomers refer to this process as active galactic nucleus (AGN) activity, and it’s different from the way stars generate energy via nuclear fusion reactions. They’re highly compact regions which have a much higher than normal luminosity over a significant portion of the electromagnetic spectrum. They’re the most luminous and persistent sources of electromagnetic radiation in the universe.