What is a black hole?
A black hole is a concentration of matter so dense that not even light can escape its gravitational pull.
What is a supermassive black hole?
Black holes come in many sizes. Astronomers believe that a “supermassive” black hole, with a mass of about 4 million Suns, lies at the center of the Milky Way galaxy. NRAO telescopes offer the best prospect for actually imaging such a supermassive black hole.
More generally, a supermassive black hole has a mass of several hundred thousand to more than ten billion solar masses. The central region of virtually every galaxy is thought to contain an object of this type. The primary evidence for supermassive black holes comes from optical and radio observations which show a sharp rise in the velocities of stars or gas clouds orbiting the centers of galaxies. High orbital velocities mean that something massive is creating a powerful gravitational field that is accelerating the stars. Additionally, X-ray observations indicate that a large amount of energy is produced in the centers of many galaxies, presumably by material falling into the accretion disk that surrounds the central black hole.
How do supermassive black holes form?
One theory is that an individual star-like black hole forms and swallows up enormous amounts of matter over the course of millions of years to produce a supermassive black hole. Another possibility is that a cluster of star-like black holes forms and eventually merges into a single, supermassive black hole. Or, a single large gas cloud could collapse to form a supermassive black hole.
Recent research suggests that galaxies and their central black holes do not grow steadily, but in fits and starts. In the beginning of a growth cycle, the galaxy and its central black hole accumulate matter. The energy generated by the jets that accompany the growth of the supermassive black hole eventually brings the in-fall of matter and the growth of the galaxy to a halt. The activity around the central black hole then ceases because of the lack of a steady supply of matter, and the jets disappear. Millions of years later the hot gas around the galaxy cools and resumes falling into the galaxy, initiating a new season of growth.
Supermassive black holes and active galaxies
If the accretion disk of the central black hole is well supplied with matter falling in from the immediate surroundings of the galactic nucleus, then it will generate large amounts of energy together with powerful jets of radiation in both directions along the rotation axis of the black hole. This gives rise to the phenomenon known as an active galactic nucleus (AGN). Quasars, BL Lacertae objects, and Seyfert galaxies are among the various manifestations of AGN, which depend on the degree of activity and the orientation of the jets of the AGN with respect to our line of sight.
Supermassive black holes and their associated accretion disks were particularly big energy producers in the early universe when galaxies were still young and their inner regions well stocked with material that could feed the central engine. In the present-day universe, most galaxies, including our own Milky Way, are much more sedate. However, AGN activity can still be rekindled if a fresh supply of matter becomes available, as for example occurs when two galaxies collide, or a large galaxy swallows a smaller neighbor.