Wednesday, April 18, 2007

Keeping warm

Image credit: NASA/CXC/IoA/A.Fabian et al.

Yesterday was a day full of meetings, most of them useful but boring. But one was our weekly colloquium, where an astronomer comes in (usually from outside the University of Texas) and talks about his or her research work, and it was quite interesting. It involves a problem that not too many people outside of astronomy have heard of, but is a big mystery -- the mystery of "cooling flows."

Cooling flows exist in large clusters of galaxies, such as the Coma Cluster. In between the galaxies in galaxy clusters, a very hot gas, tens of millions of degrees, permeates the otherwise empty space. This gas is invisible to human eyes, but it radiates a lot of light in the X-rays.

Like anything hot, the gas in galaxy clusters is cooling off. In some clusters, it is cooling so quickly that we should be able to see it changing from hot gas to cool gas and into stars, which should cause the galaxy at the center of these clusters to form stars at a rate a hundred times that of the Milky Way. But the galaxies in these clusters don't show new stars -- they are all very old.

The only explanation for this is that something is heating the gas up again before it gets too cool. But what?

Most people think it has to be a giant black hole, billions of times the mass of the sun, at the center of the galaxy cluster. These black holes are known to exist, and they often shoot giant plumes of gas and particles a million light-years or more into deep space. These black holes should produce enough energy to heat the cooling X-ray gas.

A few years ago, the Chandra X-ray Observatory found giant bubbles blown in the X-ray gas by the central black hole. The picture at the top of this post is one such galaxy cluster, and you can see several bubbles. The thought is that these bubbles, which contain a lot of energy, dissolve (or "pop") and release all of that energy into gas, perhaps as heat energy, or perhaps as sound waves.

The talk yesterday by astronomer David De Young was technical, but it dealt with the physics behind these bubbles. And, strangely, the physics seems to show that these big bubbles don't want to pop -- they want to stick together and slowly float off out of the galaxy cluster, kind of like a blob in a giant lava lamp. And if the bubbles in galaxy clusters do that, they don't transfer their energy to the X-ray gas. And so the mystery remains as to why the gas stays hot.

Maybe the physics used to explain these bubbles isn't complete -- we don't yet have the computing power to determine what happens in these bubbles over very long times, so the physicists have to make simplifying assumptions. Perhaps the black holes also emit energy in forms other than the giant bubbles, and that energy is what does the heating. We really don't know. And so, the mystery surrounding some of the most energetic things in the Universe -- giant black holes at the centers of giant clusters of galaxies, remains.

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