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Beast at galaxy core sits, lacking spin

New observations reinforce quiescent image of Milky Way's central black hole

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2:27pm, December 7, 2010
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HEIDELBERG, Germany — If it’s not as dead as a doornail, the gravitational monster at the Milky Way’s center is clearly lethargic. The galaxy’s supermassive black hole barely radiates, and certainly doesn’t hurl jets of material into space — signs that it has been on a starvation diet.

And a new study suggests that the central black hole barely spins, either.

Black hole spin doctors Avery Broderick of the Canadian Institute for Theoretical Astrophysics in Toronto and his colleagues, including Avi Loeb of Harvard University’s Institute for Theory and Computation, base their tentative findings on millimeter-wave radio emissions from the galactic center recorded by a trio of radio telescopes. Collectively, the electronically linked radio dishes act as a single giant dish, or interferometer, enabling astronomers to start homing in on the black hole’s millimeter-wave emissions. That radiation penetrates the dust and gas that concentrate in the crowded central region of the galaxy.

The emissions are believed to be generated by the material at the inner edge of the accretion disk, the whirlpool of matter that spirals into and fuels the black hole. Combing the millimeter-wave data — which is still relatively sparse —  with extensive modeling of the expected emissions suggests that the supermassive black hole spins slowly or not at all, Loeb reported December 6 at the biannual Texas Symposium on Relativistic Astrophysics. The researchers also describe their study in an article posted at arXiv.org on November 15 (http://arxiv.org/abs/1011.2770).

The apparent lack of spin indicates that the black hole has not supped sumptuously for some time, Loeb said. If it had, the rotational energy of a clump of particles devoured by the black hole would have spun the body up. A small spin doesn’t preclude that the hole snacks in fits and starts, however. During each mini dining episode, the black hole would typically ingest matter that rotated in a random direction, with the total spin averaging out to zero.

Loeb and his collaborators “have not ‘seen’ direct evidence for low spin,” noted radio astronomer James Moran of Harvard-Smithsonian, who was not part of the study. But the team has demonstrated that if a very specific set of models about the black hole is adopted along with a few assumptions, “then a case can be argued … that the spin is indeed low,” he added.

As the millimeter-wave interferometry data improve with time and more radio dishes are added to the array (SN: 10/9/10, p. 22), “the noose will close around the true value of the spin parameter,” Moran said. “I can’t wait to see the answer.”

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