One of the universe's largest structures is the galaxy cluster, in which up to 1,000 galaxies like our Milky Way are thought to have formed in the collapse of a giant cloud of primordial gas. Among the leftovers of such a colossal event was hot gas that permeated the space between galaxies within the cluster. That gas, theory predicts, ought to cool down over time, but typically it does not.            

Now researchers say they might have solved the mystery, which has vexed theorists for three decades.

Jets of enormous energy emitted by supermassive black holes reheat the gas of intergalactic space, according to a study that will be published in the July 18 issue of the journal Nature.

Christian Kaiser at the University of Southampton and Marcus Brggen of the International University Bremen in Germany used a supercomputer to simulate how intergalactic gas ought to flow and behave, and how it interacts with galaxies.

Here's what they knew at the outset:

The gas between galaxies is detected because it is hot enough to emit X-rays. But these emissions should radiate heat away and cause the gas to cool down within a few billion years, condensing to form stars and other galaxies. Yet astronomers find that in the several billion years since most galaxy clusters were born, the gas between them is still hot.

Meanwhile, residing within any galaxy cluster are some number of galaxies anchored by supermassive black holes, each of which can contain the mass of billions of stars. Some of these black holes are actively consuming matter. They'll eat anything that gets close enough, and in the process the material is accelerated to nearly the speed of light just before it is swallowed.

At this speed, the material becomes superheated. X-rays are emitted, which is how astronomers detect black holes, which otherwise can't be seen. Some of the X-rays, along with other electromagnetic radiation, are emitted in two opposing jets that stream out from the galaxy along its axis of rotation.

However, other galaxies with supermassive black holes, including our Milky Way, appear to have switched off. They're no longer emitting significant radiation. The reason behind this, too, remains puzzling.

Here's what the new simulations revealed:

When gas in intergalactic space cools, it behaves less erratically and eventually settles into a galaxy, lured there by gravity. Eventually, it cools to the point that it is absorbed by the black hole. The gas is then reheated as it spirals inward. Energy is ejected back into intergalactic space via the jets.

These jets can pack a wallop equal to 10 billion supernovae, the researchers say -- more than enough to re-heat the hot gas in galaxy clusters.

"Our results indicate that the black holes in active galaxies behave like cosmic thermostats," Kaiser said. "The hot gas in a galaxy cluster cools down to low temperatures and flows to the cluster center. There the black hole is waiting. It swallows some of the cold gas and the energy from this process drives jets into the cluster gas further out."

Gas is heated and driven from the center of the cluster, Kaiser explained, robbing the black hole of its fuel and effectively shutting it down.

"After the gas has cooled down once more, the whole cycle starts again," he said.

Kaiser and Brggen are continuing their research and hope to pin the process down more fully.