for National Geographic News
Images from the Hubble Space Telescope have revealed an important relationship between supermassive black holes and spiral galaxies: The more massive the black hole, the tighter the host galaxy's arms.
"This means that to determine the mass of a supermassive black hole, you only need an image of a galaxy," said Marc Seigar of the University of Arkansas at Little Rock.
Within the past decade astronomers have determined that almost every galaxy has a black hole lurking at its center that can range from ten thousand times to a billion times the mass of the sun.
Astronomers usually measure a supermassive black hole based on how fast stars are swirling around it—the faster the stars' orbits, the more massive an object must be present.
But that method only works for relatively nearby galaxies where individual stars can be observed, said Seigar, who announced the find today at the 212th meeting of the American Astronomical Society in St. Louis, Missouri.
For more distant galaxies, astronomers have been searching for a different technique—and looking at the positions of spiral arms could be a solution, based on comparisons of 27 spiral galaxies near enough to size up their black holes.
Using Hubble pictures, Seigar and colleagues found that galaxies with smaller black holes had angles of up to 43 degrees between the arms and the central bulge, while galaxies that harbored enormous black holes had arms at angles of about 7 degrees to their centers.
"The real significance of this is that it is relatively easy to detect spiral structure in galaxies, even out to distances of eight billion light-years," Seigar said. "So we can estimate the masses of very distant black holes now and determine how black holes grow over time."
New Hubble images are also helping astronomers study the tumultuous events that give rise to starburst and quasar galaxies.
Mike Brotherton at the University of Wyoming said the pictures provide support for a leading theory of galaxy evolution: "that galaxy interactions and mergers are a key step."
Galactic mergers are believed to trigger "starburst" periods of stellar birth, although dust and debris can shroud the new stars from view.
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