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Giant black holes just got bigger

9 June 09 14:53
By Jonathan Amos
Science reporter, BBC News

Some of the biggest black holes in the nearby Universe may be much larger than previously thought.

A reassessment of the monster hole at the core of the M87 galaxy suggests it could have 6.4 billion times the mass of our own Sun.

This result is two to three times the estimates from earlier studies.

Dr Karl Gebhardt told an American Astronomical Society meeting that his investigations led him to believe many other holes were under-recorded, also.

Most galaxies are thought to contain gargantuan central black holes. Indeed, there is thought to be a very tight correlation between the size of a galaxy and the core which consumes all matter that comes too close to it.

If a hole gets too big, too fast, however, it starts to push back, spewing high-velocity jets of matter out into space.

"There are processes that happen around the black hole that affect how much material you can dump on it," explained Dr Karl Gebhardt from 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."

But if supermassive black holes are actually bigger than previously thought, the assumptions that help describe the relationship to their host galaxies might now have to be re-evaluated, the researcher said.

Big calculations

Astronomers weigh supermassive black holes by studying the size of their host galaxies and, critically, the speed with which stars move around inside those galaxies.

The new study used novel computer modelling techniques to tease apart the relative contributions to the total mass of M87 from its visible stars, its black hole and its "dark halo".

The dark halo is a spherical region surrounding the galaxy that extends beyond its main visible structure.

It contains "dark matter", an as yet unidentified material that cannot be directly detected by telescopes but which astronomers know is there from its gravitational interaction with everything else 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, ie 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 group to incorporate the dark halo into the mass calculations and the complexity of their effort required the use of a supercomputer.

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

Distant puzzle

The results of the research, presented at the 214th meeting of the AAS in Pasadena, California, suggest that all black hole masses for the biggest galaxies may have been underestimated.

Dr Gebhardt told the meeting that this assessment was supported by data coming from the most recent - although not yet published - observations using the world's most sophisticated telescopes.

Realising that nearby supermassive black holes are actually bigger than previously thought also goes some way to solving a paradox concerning the masses of giant black holes in the distant Universe.

The black holes that power quasars - the compact but extremely luminous galaxies seen in early cosmic history - are considerably bigger than anything seen in the local Universe. This has always puzzled astronomers.

"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."

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