June 9, 2009

Astronomers Say Nearby Black Holes Larger Than Previously Believed

Some of the black holes nearest to Earth may be larger than previously believed.

A re-examination of the vast black hole at the core of the nearby M87 galaxy indicates it could have 6.4 billion times the mass of the Sun, two to three times larger than previous studies had suggested.

The reassessment has led scientists to speculate that many other black holes are also under-recorded, said Dr. Karl Gebhardt in remarks made at a meeting of the American Astronomical Society (AAS) in Pasadena, California.

Most galaxies are believed to have enormous central black holes, with a very close correlation between the galaxy's size and the core that devours all the matter that comes too close to it.  However, if a hole gets too large, too quick, it begins to push back, emitting high-velocity streams of matter out into space.

"There are processes that happen around the black hole that affect how much material you can dump on it," said Dr. Gebhardt of the University of Texas at Austin.

"Eventually, it gets so massive that the jets that are common in these galaxies blow out and they can actually halt the material that falls in."

If the black holes truly are larger than astronomers previously believed, the assumptions that describe the hole's relationship to their galaxies might need to be re-evaluated, he added.

Astronomers weigh supermassive holes by examining the size of their host galaxies and the velocity with which stars move around within those galaxies.

The new study used innovative computer modeling methods to differentiate the relative contributions to the total mass of M87 from its black hole, its visible stars and its "dark halo" -- a spherical area that surrounds the galaxy and extends beyond its visible structure.

The "dark halo" contains "dark matter", a material not yet identified that telescopes cannot directly detect.   Nevertheless, astronomers know it is there from its gravitational interaction with all the other matter that can be seen.

"In order to get the small-scale analysis correct, you have to include what the stars are doing at the outer envelope of the galaxy, i.e. you have to understand the effect of the dark halo because that is where the dark halo lives - at the edge of the galaxy."

Dr. Gebhardt and colleague Dr. Jens Thomas from the Max Planck Institute for Extraterrestrial Physics are the first team to include the dark halo into the mass calculations. The work was so complex that a supercomputer was required.

"This model took a few days [to run] whereas in the past it would have taken 10 years," Dr. Gebhardt told BBC News.

The research, which was presented at the 214th meeting of the AAS, indicates that the masses of black holes for the largest galaxies may have been underestimated.

The reassessment was supported by data from the latest observations of the world's most sophisticated telescopes, Dr. Gebhardt said.

The realization that nearby supermassive black holes are larger than previously thought may also help solve a paradox that has long puzzled astronomers about the masses of giant black holes in the distant Universe.

The black holes that power quasars - small but very luminous galaxies seen in the early history of the Universe - are substantially larger than anything seen in the local Universe. 

"By increasing masses [of local black holes] by two to three, it almost makes that problem go away," Dr. Gebhardt said.

"That is, we're beginning to resolve the differences between the masses of the black holes in quasars and the masses of black holes from nearby galaxies. That's quite exciting when things start to come together."


Image Credit: NASA


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