Tsunamis of gas hot up galaxy clusters

Tsunami-like flows of gas churn and heat the centres of galaxy clusters, suggests a new study. The turbulence may account for recent observations that the cores of clusters are unexpectedly hot.

Galaxy clusters – of hundreds or thousands of galaxies - are massive reservoirs of gas. In the 1970s, the first x-ray satellites showed the clusters' centres glowed brightly in x rays, suggesting the searingly hot cores were radiating away their energy. Astronomers believed the cores would quickly cool and contract, drawing chilled gas inward to form stars or gas clouds.

But that theory was shattered by observations. Astronomers could not find star-forming regions in the cores. And in 2001, the orbiting x-ray telescopes Chandra and XMM Newton failed to find cool gas there as well.

"It was quite a bombshell," says Ramesh Narayan, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US. "Something is heating the gas even as it is radiating."

Some astronomers believe the heating comes from jets of gas powered by black holes in the clusters' active galaxies. "But if you just stir the gas, that would transport energy from one place to another and might also do the job," Narayan told New Scientist.

Runaway process

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	Related Stories   Galaxy cluster X-rays confirm dark energy 18 May 2004   Dark matter map reveals galaxy cluster's appetite 17 July 2003   Galaxy clusters radiate gamma ray "fog" 15 August 2002     For more related stories search the print edition Archive     Weblinks   Fujita et al. paper (PDF)   Tsunami simulations   Jimmy Irwin, University of Michigan   Astro-EII, Japan Aerospace Exploration Agency

That is exactly what a team of researchers led by Yutaka Fujita at the National Astronomical Observatory in Tokyo, Japan, have found in new computer simulations. The team modelled a cluster as it sucked up gas from its surroundings over five billion years. The infusions of gas were represented by 330,000 light-year-long waves that brought hot gas from the outer core into contact with cooler, denser gas from the centre.

"It's a runaway process - the amount of swirling depends on the density of gas," comments Jimmy Irwin of the University of Michigan. Just as a tsunami swells as it moves from deep water towards the shore, he says, "you start with small turbulence in the outer regions and this is amplified".

The researchers themselves seem surprised at the complexity of the mixing. "It is interesting that the turbulence . . . is naturally caused even by the regular, wave-like, linear perturbations. No irregular triggers, which are usually essential for initiating or maintaining turbulence, are necessary," they write in a draft of the research.

Some initial signs of turbulence - x-ray spectra that suggest rotation - have been seen in some clusters already, but at distances beyond the core. Irwin, who calls the new work "promising", says the joint US-Japanese Astro-EII x-ray satellite may resolve the issue when it launches early in 2005.

Maggie McKee