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Unsuspected violence in the hearts of galaxies
Jan. 10,
2006
Courtesy Chandra X-Ray Observatory
and World Science staff
Black holes are creating havoc in unsuspected places, in the centers of galaxies, according to a new study.
The study involved an examination of images of a class of galaxies known as ellipticals, because of their shape. Elliptical galaxies tend to look calm in visible light, but the researchers found that when they observe other types of light coming from them, the picture is quite different.
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Two
galaxies seen in X-ray (blue & white) and optical (black & white)
light. The images show the shapes of the massive clouds of hot gas that produce X-ray light differ markedly from the distribution of stars that produce the optical
light, astronomers say. (Credit: X-ray: NASA/CXC/U. Ohio/T.Statler & S.Diehl; Optical: DSS)
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The giant central black holes in these galaxies is generating far-reaching explosions, they said, after analyzing the images taken by NASA’s orbiting Chandra X-ray Observatory.
What betrayed the violence was massive clouds of hot gas, stirred up by the
explosions, the astronomers said.
“These results are part of an emerging picture that shows the impact of super-massive black holes on their environment is far more pervasive than previously thought,” said Thomas Statler of Ohio University in Athens, Ohio, one of the researchers.
Added Wilt Sanders of NASA’s Chandra Program: “This is another example of how valuable it is to observe the universe at different wavelengths besides just the traditional optical wavelengths,” which characterize visible light.
A wavelength is the distance between crests of a light wave. Light can have a huge range of wavelengths; but it is only visible when its wavelength is between about
0.4 and 0.7 thousandths of a millimeter.
“Without these X-ray and radio observations, we wouldn’t know these apparently static galaxies in reality are still evolving due to the interaction with their central black holes.”
The results came from an analysis of 56 elliptical galaxies in the Chandra data archive by Statler and doctoral candidate Steven Diehl. Unexpectedly, they said, they found the multimillion-degree gas in these galaxies was distributed much from the stars, suggesting some sort of shakeup was going on.
“Most elliptical galaxies have traditionally been considered to be quiet places, like placid lakes,” Statler said. “Our results show these galaxies are a lot stormier than we thought.” About 10 percent of galaxies are classed as elliptical.
Previous X-ray studies have shown elliptical galaxies contain multimillion degree gas whose mass is a few percent of the stars in it. Under a peaceful climate, astronomers assumed the hot gas would
settle into a shape similar to, but rounder, than the stars, Diehl explained.
But “we found the distribution of hot gas has no correlation with the optical shape,” Diehl said. “Something is definitely making a mess there, and pumping energy equivalent to a supernova every century into the gas.” A supernova is a exploding, dying massive star.
Although supernovae are a possible energy source, the researchers said they identified black holes as a more likely cause of the violence. They found a relationship between the shape of the gas clouds and the power produced by energetic electrons, subatomic particles that carry electric charge. This power output can be traced to the centers of the galaxies, where super-massive black holes sit, they said.
Repetitive explosions fueled by gas sucked into central black holes is known to occur in giant elliptical galaxies
within galaxy clusters, said Diel, so his work suggests the same happens in isolated elliptical galaxies.
The team described the findings Jan. 10 at the annual meeting of the American
Astronomical Society in Washington.
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