Astronomers Detect Activity from "Quiet" Supermassive Black Holes
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453April 13, 1999
Charleston, S.C. -- Astronomers have heard the first shy words from seemingly quiet supermassive black holes in the form of a unique type of X-ray light. These black holes exist in the centers of the oldest, largest galaxies and have a mass of about a billion suns, compressed into a region comparable to the size of our solar system.
While a small percentage of supermassive black holes generate huge X-ray glows (in a phenomenon known as an active galactic nucleus, or AGN) the vast majority of supermassive black holes are quiescent, producing no detectable X-rays. They are known only through their gravitational effects on star and gas rotation.
These new observations suggest that quiescent supermassive black holes do produce X-rays, but the emission is much feebler than that observed in AGN and with a different kind of light spectrum. The new results add credence to the ubiquity of supermassive black holes, which are likely to exist in the cores of all galaxies, including our own. Also, the unique X-ray light may provide clues to the origin of Universe's still unexplained X-ray background radiation.
Dr. Tiziana Di Matteo of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and Dr. Steven W. Allen of the Institute of Astronomy in Cambridge, UK, present these results at the High Energy Astrophysics Division of the American Astronomical Society meeting in Charleston, S.C., April 13, 1999.
"We have found that these giant black holes, lurking at the centers of nearby galaxies, are not totally hidden from us, but manifest themselves by radiating small amounts of very energetic X-ray light," said Dr. Di Matteo. "We cannot explain this light as being due to any other type of source."
Dr. Allen said the key evidence was the type of light: energetic X-ray light with shorter wavelengths. The X-ray light that Drs. Di Matteo and Allen observed in six nearby old galaxies was relatively more energetic compared to light associated with very luminous black holes in AGN and quasars, albeit in smaller quantities. The differences in light characteristics, Dr. Di Matteo said, might be due to the different ways in which matter falls into these supermassive black holes.
Matter does not fall directly into a black hole, but rather spirals in -- like water down a drain -- through what is known as an accretion disk. In younger, spiral-shaped galaxies, matter accumulates into a dense disk; particles collide frequently and radiate X-rays, keeping the temperature of the disk at millions of degrees and the disk relatively thin. Such is the case for AGN.
In older, elliptical galaxies, such as the six that Drs. Di Matteo and Allen studied, the black hole accretion disk accumulates matter at a slower rate and collisions are much less frequent, causing the disk to be tenuous and bloated and the matter in the disk to grow very hot. The result is a much less luminous black hole region that converts the available energy into radiation much less efficiently.
"This is a wonderful solution for the problem of dim black holes," said Dr. Di Matteo. "The spectrum of radiation, ranging from high energy X-rays to radio waves, emitted from the centers of these six elliptical galaxies, matches that predicted by models of such low-efficiency accretion."
The astronomers note that it is very interesting that this "low-efficiency" mode of accretion is occurring in older galaxies. The abundance of newborn galaxies in the early Universe, they said, would have provided black holes with plenty of gas to feed on, enough to generate the light of quasars. As these galaxies matured, they converted their gas into stars, limiting the fuel available to the central black holes. With less fuel to feed their black holes, low-efficiency accretion takes over. Today, the cores of galaxies that were once ablaze with quasar light find themselves in the dark ages. Drs. Di Matteo and Allen's results further corroborate the idea that the present-day Universe may therefore be rife with dead quasars.
The astronomers were enthusiastic about the launch later this year of NASA's Chandra Observatory, a revolutionary X-ray astronomy satellite, which astronomers hope will help provide clear answers to the nature and origin of the X-ray light from supermassive black holes in the centers of galaxies.
Drs. Di Matteo and Allen utilized data collected by the Advanced Satellite for Cosmology and Astrophysics, a Japanese X-ray satellite launched in 1993 with a Japanese and NASA payload.
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