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Turbulence responsible for black holes' balancing act
Posted: JULY 15, 2009

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New simulations show that star formation in some galaxy clusters is prevented by turbulence created by jets of material leaping from the discs of the black holes at galaxy centres.

Galaxy clusters are the largest structures in the Universe, containing hundreds or even thousands of galaxies. Although well studied, it has long been speculated as to why the gas at the centre of some of these clusters is not forming stars, even though it is rapidly cooling and condensing.

A new model, devised by Evan Scannapieco from Arizona State University (ASU) and Marcus Brueggen of Jacobs University in Germany, may have the answer.

"There are two types of clusters: cool-core clusters and non-cool core clusters," explains Scannapieco. "Non-cool core clusters haven’t been around long enough to cool, whereas cool-core clusters are rapidly cooling, although by our standards they are still very hot."

Snapshot of gas temperatures in a three dimensional computer simulation of a cool-core cluster. The blue ring shows the cool gas accreting onto the central black hole disc; the red and yellow jets show the hot gas ejected by this disc. Older bubbles from an earlier outburst are visible on the far left and right sides of the image. Image: E. Scannapieco/ M. Bruggen / ASU Fulton High Performance Computing Initiative.)

Cool-core clusters provide the focus of the new study. X-ray observations show that the hot diffuse gas making up the clusters, known as the intracluster medium, is rapidly cooling into the hearts of cool-core clusters. Lurking at the centre of each galaxy, however, is a supermassive black hole. These monsters gobble up some of the cooling gas, while the rest is ejected in regular jets from the swirling disc of material surrounding the black hole.

This may all sound relatively normal behaviour for a black hole environment, but the regularity of the outbursts and the failure of the gas clouds to drop to a cool enough temperature to form stars presented a mystery, until now.

"It looked like the jets coming from black holes were somehow responsible for stopping the cooling," says Scannapieco, "but until now no one was able to determine how exactly."

Scannapieco and Brueggen used ASU's supercomputers to develop a three-dimensional simulation of a galaxy surrounding a massive black hole, based on a previous simulation devised by Guy Dimonte at Los Alamos National Laboratory and Robert Tipton at Lawrence Livermore National Laboratory. The new model, however, added a vital missing ingredient: turbulence. Brueggen likens the turbulence to the same way that rising thunderstorm clouds on the Earth produce turbulence in the atmosphere.

Without accounting for turbulence, the jets emanating from the black hole would grow stronger and stronger, and the gas would cool rapidly into into new stars. Including turbulence in the models balances this cooling action by mixing and heating the surrounding gas so that it does not accrete onto the black hole. The jet stops and there is nothing to drive the turbulence so it fades away. As a result, the hot gas no longer mixes with the cold gas, so the centre of the cluster cools, and more gas is captured by the black hole. At this point another jet is born, which mixes the gas together, and the cycle is repeated.

Could elliptical galaxy M87's black-hole-powered jet be due to the turbulence that Scannapieco and Brueggen describe? Image: NASA/STScI)

"When you have turbulent flow, you have random motions on all scales. Each jet of material ejected from the disc creates turbulence that mixes everything together," says Scannapieco. "The time it takes for the turbulence to decay away is exactly the same amount of time observed between the outbursts.”

Brueggen tells Astronomy Now that this kind of turbulent feedback might explain the features observed in the largest known galaxies like M87. One way to confirm the results of the model, he says, would be to measure black hole jets using X-ray spectroscopy, such as with the planned X-ray satellite ISO (International X-ray Observatory), although not planned to launch until the 2020s, astronomers will have a long time to wait.

The new model is presented in a forthcoming edition of the journal Monthly Notices of the Royal Astronomical Society.

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