supermassive black hole
The current heavyweight champion of known supermassive black holes is that at the center of a quasar called OJ287, weighing in at 18 billion solar masses. This quasar lies about 3.5 billion light-years away in the constellation Cancer. The mass of its central black hole, which is six times greater than that of the next nearest known rival, has been measured quite accurately because there is a second massive black hole in the heart of the OJ287 system. The smaller black hole, which has a mass of only about 100 million Suns, orbits around its much greater partner at a distance of a mere 1.5 light-years.
How supermassive black holes formOne 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, it may be that a single large gas cloud collapses to form a supermassive black hole.
Recent research, including results from the Chandra X-ray Observatory, suggests that galaxies and their central black holes don't grow a steady rate, 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.
Clues from galactic loners
One such system is NGC 4178 – a spiral galaxy with a flat disk and no central bulge (see photo). It has apparently led a solitary existence, so that its small central black hole is either the size it started out with or has been increased only modestly by occasional in-falls of matter from the immediate surroundings. No great influx of material has come from other galaxies or from mergers with other black holes. This makes the exact size of NGC 4178's central black hole of great interest because it is essentially a survivor of the kind of black holes that must have been common in the early universe and that were found in the first galaxies.
Using data from Chandra, astronomers have estimated NGC 4178 to have a central black hole weighing between about 10,000 and 100,000 solar masses, making it one of the lowest-mass nuclear black hole currently known. The fact that several such pristine galaxies turn out to have a small, still-expanding black hole at their core suggests that black holes can grow to intermediate size without mergers, but then need to pool their resources to get much bigger.
Supermassive black holes and active galaxiesIf 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.
WISE reveals millions more supermassive black hole
From its observations, astronomers have identified about 2.5 million actively feeding supermassive black holes, at distances stretching out to 10 billion light-years. About two-thirds of these objects were previously unknown because dust in the accretion disks that surround them blocks visible light, although it glows brightly in the infrared.
Related categories• BLACK HOLES
• SPACE AND TIME
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