Black Holes Have Been Underestimated

9 June 2009 (All day)

Tim Jones/University of Texas, Austin, after K. Cordes & S. Brown/STScI

Bigger than ever. New modeling shows that black holes at the center of galaxies are far more massive than previously thought.

PASADENA, CALIFORNIA--Astronomers peering into the heart of one of the largest galaxies in our neighborhood have found that the black hole at its center is two to three times more massive than previously thought, making it the biggest black hole to have ever been measured in the nearby universe.

The result, reported here yesterday at the semiannual meeting of the American Astronomical Society, suggests that researchers have been severely underestimating the masses of black holes within nearby galaxies. The finding is likely to compel researchers to revise their views about the mechanics of galaxy formation around black holes.

Researchers figure out the mass of a galaxy by measuring the velocity of the stars orbiting within it and calculating the strength of the gravitational pull required to keep them moving at that speed. These calculations also help scientists determine the mass of black holes, which lie at the center of nearly all galaxies.

Karl Gebhardt of the University of Texas, Austin, and Jens Thomas of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany applied new computational models to work out the mass of the black hole in the middle of M87, a giant galaxy 50 million light-years away. What the researchers did differently from previous estimations was factor in the dark halo--the invisible sphere of dark matter surrounding the galaxy--which astronomers had believed to be only a relatively small component of the overall mass. Another reason that Gebhardt and other researchers did not attempt to incorporate the dark halo in previous models was the huge amount of computing power needed for the job.

Running the model on a supercomputer, the team found the mass of M87's black hole to be 6.4 billion times the mass of our sun--two to three times larger than previous estimates. The model provided similar results for four other nearby galaxies, multiplying their black hole masses by factors of 1.5 to 3.

The results, which will be published in The Astrophysical Journal later this summer, "caught us off guard," says Gebhardt. "It looks like we've been systematically underestimating the mass of black holes."

Because the gravity of the black hole influences the behavior of the galaxy around it, a doubling or tripling of the black hole mass will alter the picture of how galactic material might be swirling around it, says Tod Lauer, an astronomer at the National Optical Astronomy Observatory in Tucson, Arizona. And that influences the way galaxies grow and change.

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