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Astronomers zero in on Milky Way's black hole

One of most precise values for its mass includes an estimate that Earth is 27,000 light-years from galaxy's center

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12:25pm, December 11, 2008
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VANCOUVER, Canada — German astronomers monitoring the motions of 28 stars at the center of the Milky Way galaxy have reported a new, more precise value for the mass of the supermassive black hole believed to lurk there.

The black hole weighs the equivalent of 4.31 million suns, with an uncertainty of plus or minus 0.36 million, Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, reported in Vancouver December 10 at the Texas Symposium on Relativistic Astrophysics. The observations also pinpoint the distance from the Earth to the galaxy’s center at 27,000 light-years.

The findings will appear in an upcoming Astrophysical Journal and currently appear online at http://arxiv.org/abs/0810.4674.

During his talk, Genzel proclaimed that the new mass and its precision are evidence for the supermassive black hole’s existence, evidence that extends “beyond any reasonable doubt.”

But Andrea Ghez of the University of California, Los Angeles, who leads a rival team studying the galaxy’s center, notes that for years both groups have reported precise enough numbers to offer compelling evidence that the galaxy's center is a black hole.

The orbits of stars whipping about the core indicate such a mass, and also suggest that this immense concentration of matter must fit into such a small volume that it would quickly crunch down into a black hole.   

Now, says Ghez, sharper optics and more observations present both teams with new obstacles in making more precise measurements. Ironically, with the current estimates for the mass of the central black hole, “we have more maturity in our answer, but less certainty.” Before, she says, “we were acting like teenagers, being very emphatic with very little information.”

Ghez and her colleagues describe some of the new challenges in the December 20 Astrophysical Journal.

Among the issues, she says, are concerns about distinguishing radiation emitted by stars from light emitted by matter falling onto the putative black hole. Confusing the two radiation sources could throw off calculations of a star’s orbit, which would make the calculated mass of a black hole more uncertain.

Another problem, cited by both Ghez and Genzel, is the lack of a good reference frame at the galaxy’s center. From year to year, as the researchers more carefully track the orbits of stars at the core, it’s unclear how to gauge stellar motion when all the stars in the image are moving and there’s no fixed frame of reference. “It’s like taking images while on a merry-go-round,” says Ghez. Such details didn’t matter as much when observations were not as sharp, she adds.

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