24 August 2000
The newly discovered object, located in the core of galaxy NGC 4395, is part of an emerging class of mid-mass black holes, a thousand times more massive than stellar black holes yet a thousand to a million times smaller than the largest variety.
Dr. Kazushi Iwasawa and Prof. Andrew Fabian at the Institute of Astronomy in Cambridge, UK, along with other astronomers from the United Kingdom and Japan, identified the mid-mass black hole using the Advanced Satellite for Cosmology and Astrophysics (ASCA), an X-ray telescope built by Japan. Their results have been accepted for publication in the Monthly Notices of the Royal Astronomy Society.
"Recent studies with Hubble and other telescopes show that massive dark objects appear ubiquitous at the centres of galaxies," said Dr. Iwasawa. "These dark objects are likely black holes, and we are beginning to learn that they come in a variety of sizes."
Supermassive black holes contain the mass of millions to billions of Suns confined to a region no larger than our solar system. They are thought to be the engines behind the bright, concentrated emission in the cores of many galaxies, such as quasar galaxies. The tremendous force of their gravity pulls in gas, dust and whole stars with such fury that the in-falling matter heats to millions of degrees and radiates brilliantly across the spectrum, particularly in X rays.
In galaxies lacking a bright core, such as our own Milky Way galaxy, scientists speculate that the central black hole has either run out of fuel -that is, has already accreted all the matter within its gravitational clutches - or, for whatever reason, is accreting matter at a slower rate. The theory of Advection-Dominated Accretion Flow (ADAF) states that matter may flow into the black hole at an expected rate, but that the radiation is emitted inefficiently, perhaps being pulled into the black hole itself.
Within the past year, scientists have uncovered mid-mass black holes. These, such as the black hole suspected in NGC 4395, contain the mass of 10,000-100,000 Suns. A black hole of such mass could explain the type of emission from low-luminosity Seyfert galaxies. These are galaxies with active cores and bright emission, albeit much dimmer than quasars or other extremely bright active galaxies.
The X-ray emission from NGC 4395 had been a mystery. Earlier observations suggested that perhaps an extremely bright group of stars was the cause for the emission. Yet the source was too bright for this stellar model. Likewise, the type of fluctuation in the X-ray signals from the source was not in sync with the ADAF black hole model.
"We now see that the nuclear source in NGC 4395 is a scaled-down version of a black hole found in the most luminous of galaxies," said Fabian. "Everything is the same, only it is smaller."
The apparent small black hole mass of NGC 4395 suggests that X-ray variability in active galaxies correlates with mass and not directly with luminosity, as previously thought, Iwasawa said. This therefore suggests that some active galaxies with low luminosity may have mid-mass black holes.
"We do not know how common galaxies like NGC 4395 are, but it certainly wouldn't be surprising if there are many out there," said Dr. Iwasawa. "NGC 4395 is the only one as far as we know. They might be just difficult to find because they tend to reside in tiny galaxies. On the other hand, many 'big' galaxies with low luminosity have been found to have a big black holes and for some reason, they accrete in an inefficient mode."
Scientists contributing to this work include Drs. Omar Almaini, Paulina Lira and Prof. Andrew Lawrence at the Institute for Astronomy at University of Edinburgh, Dr. Kiyoshi Hayashida of Osaka University, and Prof. Hajime Inoue of Japan's Institute of Space and Astronautical Science. Dr. Lira has since joined Leicester University.
ASCA, launched in February 1993, is Japan's fourth cosmic X-ray astronomy mission, and the second for which the United States has provided part of the scientific payload. The satellite is operated through NASA's Goddard Space Flight Center in Greenbelt, Maryland.
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