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Supernova shock wave may regulate activity in galaxy center

Scientists from MIT and other institutions using NASA's Chandra X-ray Observatory have discovered an apparent supernova remnant in the center of our galaxy. They believe this remnant might help regulate the supermassive black hole at the center of our galaxy and that such relationships between supernova remnants and black holes might be common throughout the universe.

The scientists studied Sagittarius A East (Sgr A East), a shell-like structure nearly 25,000 light years from Earth in the constellation Sagittarius. Sgr A East surrounds the supermassive black hole known as Sagittarius A* (denoted with an asterisk because it is a point source that emits radio waves), which is offset by about six light years from the center of Sgr A East.

Using Chandra, scientists were able to separate Sgr A East from other complex structures for the first time in X-ray wavelengths. The properties they discovered support the long-standing hypothesis that Sgr A East is a single supernova remnant that exploded about 10,000 years ago.

"With Chandra, we found a hot gas concentrated in the larger radio shell of Sgr A East," said Yoshitomo Maeda of Pennsylvania State University, who presented the research team's results at the 197th national meeting of the American Astronomical Society in San Diego on January 10. "The gas is highly enriched by heavy elements, with four times more calcium and iron than normal solar abundances, indicating Sgr A East is most likely a remnant of a supernova explosion."

After the explosion, scientists believe a shock wave heated gas to temperatures of 20 million degrees and that the gas and shock wave helped shape activity at our galactic center. Scientists believe two shocks, one moving inward and one outward, were formed and driven by the supernova ejecta. The inward shock wave heated up the ejecta that was detected with X-rays by the Advanced CCD Imaging Spectrometer (ACIS) aboard Chandra. Scientists believe the outward shock wave moved the cooler, heavier ambient gas that comprises the intergalactic medium, compressing and plowing that gas past the black hole as the shock wave spread and feeding the black hole in the process.

"An important question to be raised here is what effect the plowed gas has on its environment," said Frederick Baganoff, an MIT research associate and lead scientist for Chandra's Galactic Center project. "It is possible that the plowed gas has passed over the supermassive black hole at some time in the recent past. During the passage, a lot of gas could have been captured by the black hole."


When black holes pull matter into themselves, they are able to accelerate those particles to almost the speed of light. Following principles similar to that of a hydroelectric power station, the matter accreting onto a black hole releases a great deal of energy, much of it in X-rays that can ionize the cooler ambient gas in the vicinity of the black hole.

"Actually, radio astronomers already found that the gas in a halo surrounding Sgr A East and the supermassive black hole is largely ionized," said Mark Morris of UCLA. "If the gas plowed by the supernova remnant was pushed past the black hole, the spectacular interaction would very possibly have occurred as recently as a few hundred years ago, and the resulting flash of energy would likely have irradiated and ionized the surrounding gas. This could explain why the ionization of the gas still survives."

In that manner, the activity of the black hole might be regulated by the supernova remnant. In a broader sense, that activity might serve as a model for other black holes and other phenomena throughout the universe. Many scientists believe massive black holes thrive at the center of most galaxies.


The Chandra observations were made September 21, 1999 using ACIS, a sophisticated version of the CCD detectors commonly used in digital cameras or video cameras, conceived and developed for NASA by Penn State and MIT. Along with Dr. Baganoff, other MIT researchers involved in this research are Mark Bautz, John Doty and George Ricker, all of the Center for Space Research. The research is funded by NASA.

The Chandra X-ray Observatory is the third of NASA's "great observatories," following the Hubble Space Telescope and the Compton Gamma-Ray Observatory.

A version of this article appeared in MIT Tech Talk on January 10, 2001.

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