Hundreds of Exotic, New Black Holes May Be All Around Us
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453June 5, 2001
Pasadena, Calif. -- Nearly a quarter of nearby galaxies have regions that may contain "intermediate-mass" black holes, a new class of objects uncovered two years ago. The shear number of these objects raises their status from a mere anomaly to a commonplace phenomenon that may have important implications for galaxy formation.
Drs. Andrew Ptak of Carnegie Mellon University and Edward Colbert of Johns Hopkins University have compiled a list of over 200 of these objects, which astronomers call Intermediate-Luminosity X-ray Objects (IXOs). They speak of their survey results, the largest to date, in a press conference today at the 198th meeting of the American Astronomical Society in Pasadena, California.
"It is difficult to make black holes much bigger than the Sun except for the supermassive ones, which probably grew to be that size from the earliest days of the Universe," said Ptak. "We don't know when and how intermediate mass black holes form, but we now have a large list to examine what these objects are in detail."
The results are a culmination of a six-month effort surveying some 750 galaxies with archived data from the German-led ROSAT satellite. A survey performed with a much smaller set of Chandra data has found similar results.
IXOs are compact sources outside of the galactic center that glow with the luminosity of a million suns. They are brighter than most supernovae, and many astronomers believe these objects are massive black holes pulling in that gas that surrounds them. As such, each IXO is a potential intermediate-mass black hole.
Before the discovery of IXOs, astronomers knew of two main types of black holes, the stellar and supermassive varieties. Stellar-sized black holes form from collapsed stars that were once at least 8 times more massive than our Sun. Once formed, these black holes have a mass about equal to the Sun.
Supermassive black holes, thought to power quasars, have the mass of millions to billions of suns confined to a region about the size of our Solar System. Astronomers say that the cores of most galaxies contain a supermassive black hole that may or may not be actively pulling in gas.
Intermediate-mass black holes have masses ranging from 10 to 10,000 suns. Colbert and Ptak, independently, were the first to report on intermediate-mass black holes at an AAS press conference in April 1999.
The ROSAT survey covered 1,500 data sets for about 750 galaxies within 200 million light years from earth. Ptak and Colbert found that about 1 in 10 of these galaxies had IXOs with luminosities consistent with that of a 100 solar-mass black hole and about 1 in 4 have fainter IXOs with luminosities consistent with that of a 10 solar-mass black hole. Some galaxies have more than one IXO.
Scientists aren't clear on whether the Milky Way galaxy contains intermediate-mass black holes. There are a few black holes in our Galaxy called microquasars that might be considered IXOs. Also, X-ray binaries -- a healthy star orbiting around a black hole or neutron star -- can be around 10 solar masses, at the low end of what an IXO is considered to be. These objects are difficult to observe because of the obscuring dust and gas in our Galaxy's disk.
The two scientists hope to examine the new catalog of IXOs with Chandra and other telescopes to get a better understanding of what they are in relation to normal X-ray binary star systems in our galaxy. Although ROSAT is not as sensitive as Chandra, the ROSAT survey covered far more galaxies than Chandra has so far.
"Never before has there been a complete accounting of the numbers of these peculiar objects," said Colbert. "Now we have a large database to use for follow-up observations and we can begin to search for clues as to what they are."
ROSAT -- short for Röntgen Satellite and named after Wilhelm Conrad Röntgen, the discoverer of X-rays -- was an X-ray observatory developed by Germany, Britain and the United States; launched by NASA in 1990; and operated by the Max Planck Institute for Extraterrestrial Physics near Munich. Observations concluded in 1999, though much archived data still awaits discovery.
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