Above: M87 is a type of galaxy that looks very different from our own Milky Way. Even for an elliptical (egg-shaped) galaxy, M87 is peculiar. It is much larger than an average galaxy and shows an unusually high number of globular clusters. Globular clusters are gravitationally bound concentrations of approximately ten thousand to one million stars, spread over a volume of several tens to about 200 light years in diameter. These globular clusters are visible as faint spots surrounding the bright center.
In general, elliptical galaxies contain similar numbers of stars as spiral galaxies, but are ellipsoidal in shape (spiral galaxies are mostly flat), have no spiral structure, and little gas and dust.
M87 is 120,000 light-years in diameter (larger than our own Milky Way which is approx. 100,000 light-years in diameter) and is the dominant galaxy at the center of the Virgo Galaxy Cluster, some 50 million light-years away. It is the likely home to a supermassive black hole responsible for the high-energy jet of particles emerging from the galaxy's central region.
Click on the image for larger view.
Report by Ker Than for National Geographic News [edited by Paul]
A new computer model suggests that the supermassive black hole at the heart of the 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 are enveloped in clouds of mysterious dark matter.
But previous models of galaxy mass did not include dark halos because calculating their masses would have used 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 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 two to three times larger, Gebhardt said, the problem "almost goes away."
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