Astronomers distinguish between three classes of black holes: Stellar-mass, Intermediate-mass and Supermassive. Stellar-mass black holes have masses from about one to 10 times the mass of the Sun, and are the end products of the evolution of very massive stars. The supermassive ones are found in the nuclear regions of galaxies. A new category called Intermediate-mass black holes has been proposed in 1999 to cover light-weight supermassives which are much heavier than stellar-massed ones, but less than a few million solar masses. We don't know how these are produced since only a few are known based on surveys by the Hubble Space Telescope.
We don't exactly know how they form, but surprisingly the Hubble Space Telescope has discovered that the masses of these supermassive holes are 0.1 percent of the mass of the nuclear region of the host galaxy. If the supermassive holes were randomly created, there would be no such rule of thumb to account for their masses. Clearly there is something, or some process, that causes the masses of these supermassive holes to get regulated by the mass of the stellar system they inhabit. Here's a recent press release from HST:
Announcing the discovery of three black holes in three normal galaxies, an international team of astronomers suggests nearly all galaxies may harbor supermassive black holes which once powered quasars (extremely luminous nuclei of galaxies), but are now quiescent.
This conclusion is based on a census of 27 nearby galaxies carried out by NASA's Hubble Space Telescope and ground-based telescopes in Hawaii, which are being used to conduct a spectroscopic and photometric survey of galaxies to find black holes which have consumed the mass of millions of Sun-like stars. The findings, being presented today at the 189th Meeting of the American Astronomical Society in Toronto, Canada, should provide insights into the origin and evolution of galaxies, as well as clarify the role of quasars in galaxy evolution.
The key results are:
1...Supermassive black holes are so common, nearly every large galaxy has one.
2...A black hole's mass is proportional to the mass of the host galaxy, so that, for example, a galaxy twice as massive as another would have a black hole that is also twice as massive. This discovery suggests that the growth of the black hole is linked to the formation of the galaxy in which it is located.
3...The number and masses of the black holes found are consistent with what would have been required to power the quasars.
"We believe we are looking at "fossil quasars" and that most galaxies at one time burned brightly as a quasar," says team leader Doug Richstone of the University of Michigan, Ann Arbor, Michigan. These conclusions are consistent with previous Hubble Space Telescope observations showing quasars dwelling in a variety of galaxies, from isolated normal-looking galaxies to colliding pairs. Two of the black holes "weigh in" at 50 million and 100 million solar masses in the cores of galaxies NGC 3379 (also known as M105) and NGC 3377 respectively. These galaxies are in the "Leo Spur", a nearby group of galaxies about 32 million light-years away and roughly in the direction of the Virgo cluster.
Located 50 million light-years away in the Virgo cluster, NGC 4486B possesses a 500-million solar mass black hole. It is a small satellite of the galaxy M87, a very bright galaxy in the Virgo cluster. M87 has an active nucleus and is known to have a black hole of about 2 billion solar masses. Though several groups have previously found massive black holes dwelling in galaxies the size of our Milky Way or larger, these new results suggest smaller galaxies have lower-mass black holes, below Hubble's detection limit. The survey shows the black hole's mass is proportional to the host galaxy's mass. Like shoe sizes on adults, the bigger the galaxy, the larger the black hole.
The 1998 list of candidates for supermassive black holes which have masses between 1 million and a few billion times the mass of the Sun includes:
Cygnus-A >1 billion solar masses Centaurus-A >1 billion solar masses Messier 87 2 billion solar masses NGC 4649 2 billion solar masses NGC 4486b 500 million solar masses NGC 4291 200 million solar masses NGC 4261 >100 million solar masses ? NGC 3115 >100 million solar masses ? NGC 3379 50-100 million solar masses NGC 3377 50-100 million solar masses NGC 4594 >100 million solar masses? Messier 106 36 million solar masses NGC 2798 20 million solar masses Messier 51 > 10 million solar masses? Milky Way's Galactic Center 2.6 million solar masses NGC 2457 2 million solar masses Messier 32 > 1 million solar masses?
The list of candidates for stellar-mass black holes includes:
A0620-00 3 - 4 solar masses Cygnus X-1 4 - 8 solar masses Cal 87 ( LMC ) 5 - 8 solar masses Sco X-1 3 - 10 solar masses GS2000+25 3 - 10 solar masses GX339-4 3 - 10 solar masses GS2000+25 3 - 10 solar masses V404 Cygni 8 - 12 solar masses Nova Muscae 1991 3 - 10 solar masses The list of Intermediate-mass Black holes: 1E1740.7-2942 100 - 10000 solar masses
Copyright 1997 Dr. Sten Odenwald
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