Most Massive Black Holes Heavier Than Thought

Ker Than in Pasadena, California
for National Geographic News
June 9, 2009

The black hole heavyweight of the nearby universe could be an even heftier behemoth than previously believed.

A new computer model suggests the supermassive black hole at the heart of the giant galaxy M87 weighs the same as 6.4 billion suns—two to three times heavier than previous estimates.

The model is the first to make detailed calculations of the "halo" of dark matter surrounding M87.

Based on their gravitational behavior, all galaxies are thought to have very massive black holes at their cores and be enveloped in clouds of mysterious dark matter.

But previous models of galaxy mass did not include dark halos, because to calculate their masses would have taken up too much computing power.

Now, using a supercomputer at the University of Texas at Austin, astronomer Karl Gebhardt and colleagues have re-run the numbers for M87.

The new estimate suggests that a large bulk of the mass initially thought to belong to stars at M87's core is actually locked up in the halo at the galaxy's outer edge.

But the actual mass of the core is still thought to be the same. So if the extra mass isn't tied up in stars, it must belong to the supermassive black hole, Gebhardt explained.

"By including the dark halo, you reduce the amount of the mass [of stars in the center of] of M87 by a factor of two," he said.

"In order to make up for that, you have to increase the mass of the black hole."

Gebhardt suspects the mass estimates for supermassive black holes in other nearby galaxies are also off by factors of two or more.

Black Hole Puzzle

If confirmed, the finding could help solve a cosmic mystery concerning the masses of central black holes in quasars.

Quasars are extremely distant galaxies that have huge amounts of radiation spewing from their core black holes.

Other studies have suggested that the "active" black holes in quasars weigh as much as ten billion times the mass of the sun. So astronomers have been puzzled as to why the central black holes in local galaxies are puny by comparison.

But if the masses of nearby black holes are actually each two to three times larger, Gebhardt said, the problem "almost goes away."

Findings presented today at the 214th meeting of the American Astronomical Society in Pasadena, California.




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