May 16, 2000 Scientists studying a nearby galaxy have found evidence of a supermassive black hole similar in size to one believed to lie in our own Milky Way. They've also observed a swirl of matter being pulled into the hole's center, revealing details about the object's mass that could shed light on how these celestial phenomena are formed.
The research, published in the latest issue of The Astrophysical Journal Letters, is part of a project by an Ohio University astronomer and an international team seeking evidence for black holes in 20 different galaxies using spectra from the Hubble Space Telescope.
Galaxies are made up of two parts -- a flat, disk-like area and a rounded area called a bulge. Many scientists suspect every galaxy contains at its center a supermassive black hole with a mass of 10 million to one billion times the mass of the sun. Some studies suggest the mass of these supermassive black holes are related to the size of the rounded part of the galaxy, leading scientists to assume that the bulge and the black hole formed together.
But this newly identified black hole has a smaller mass than expected for its galaxy's bulge, suggesting they formed at different times. And because this black hole is similar in size -- up to 10 million times the mass of the sun to one thought to inhabit the Milky Way, scientists say what they're learning can be applied a little closer to home.
"The kind of phenomenon we see in this object may be very common, and it may be that our galaxy was once in a similar phase," said Joseph Shields, an assistant professor of physics and astronomy and lead author of this newly published research. "The formation of a supermassive black hole is a dramatic event in the history of a galaxy and may have important consequences for how that galaxy evolves."
Shields studies quasars, which prompted his interest in the study of black holes. Matter falling into black holes most likely serves as an energy source for quasars, objects that resemble stars but emit far more light. Gravity from a black hole causes matter around it to speed up as it falls into the hole's center. Gas, dust and other materials bump into each other as they swirl madly, causing them to heat up and give off a very bright glow, which astronomers observe as a quasar.
With the Hubble's lens directed at a galaxy called NGC 4203 located some 30 million light years from Earth, Shields and his colleagues were able to capture data on a ring of matter, called an accretion disk, moving along the edge of a black hole.
"There is light coming from the center of this galaxy and it's evidently not being produced by stars," Shields said. "We think the light is coming from matter that is falling into the black hole, which provides evidence that this black hole is probably a faded quasar."
As the matter succumbed to the black hole's gravitational pull, it was heated to a temperature as high as 100 million degrees, which changes the structure of the accretion disk. Charting those structural changes helps scientists determine just how much gas and other matter is falling into the hole, which Shields said is a crucial element in understanding what powers quasars.
The next step in this project, which is supported by NASA and the Space Telescope Science Institute, will be to obtain more spectra of the accretion disk surrounding NGC 4203, which will allow Shields to calculate the black hole's exact mass. Then he and his colleagues will turn their attention to the study of the 19 other galaxies included in this project, all of which offer evidence for black holes.
"Black holes are phenomenal objects," Shields said. "They naturally inspire curiosity."
Written by Kelli Whitlock.
Attention Editors: A 72 dpi image of an accretion disk is available for viewing at http://www.ohio.edu/researchnews/pix/pages/BLACKHOLE_low.htm. For a video on black holes, visit http://universe.gsfc.nasa.gov/realmedia/bhjourney/bhjourney.ram. For a copy of the journal article on which this release is based, contact Kelli Whitlock at 740-593-2868.
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