Supermassive Black Hole Kicked Out of Galaxy

Every respectable galaxy has a supermassive black hole at its center -- all except the one galaxy that has now been caught in the act of ejecting its insatiable mass-muncher. To figure out how this galaxy managed it, supercomputers were put to work.

They found that the ejection is probably the result of two mega black holes merging as part of a galaxy-on-galaxy collision.

"Since most sizable galaxies host massive black holes in their cores, mergers between such sizable galaxies should result in the merger of the massive black holes in their centers as well," explained Peter Jonker of Texas A&M University.

And that merger of black holes, each with masses equivalent to millions or billions times our sun, can create an unusual sort of recoil that sends the newly merged black hole on a trajectory to intergalactic exile.

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The discovery was made when undergraduate student Marianne Heida of the University of Utrecht, was comparing hundreds of thousands of cosmic sources of x-rays, as charted by the orbiting Chandra X-Ray Observatory, with the positions of millions of galaxies.

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Normally the bright X-ray points found by Chandra are at the centers of galaxies, since it's there that super hot material falling into the central black hole radiate copious X-rays.

What Heida found was a bright X-ray point that was not centered on the known galaxy. That suggested a black hole had been ejected. This discovery has now made the pages of the journal Monthly Notices of the Royal Astronomical Society.

The ejection process, as simulated by supercomputers, involves such exotic things as gravitational waves to help propel the two merging black holes out of a galaxy. Gravitational waves are fluctuations in the curvature of space and time which propagate through the universe as gravitational waves -- outward from its source.

They were predicted by Albert Einstein as part of his theory of general relativity and expected to be generated whenever extremely dense and massive objects get too close to each other.

"The idea of ejection is of great interest because you have the possibility of gravitational waves coming from the recoil (of the merger)," said black hole researcher Cole Miller at the University of Maryland. "As (the black holes) spiral in, they create gravitational waves."

In fact they must release energy in that way in order to degrade their orbits and get close enough to merge. Those gravitational waves could act sort of like a jet engine, explained Jonker.

"The gas accelerated and expelled by the jet turbine causes the plane to move forward," said Jonker, "Similarly, the gravitational wave emission in the final stage of the merger process is emitted preferentially in one direction, causing the newly formed black hole to be propelled in the other direction."

"In principle you should get a whale of a kick," agreed Miller. "So (this discovery), though very indirect, is the kind of thing that tests Einstein's theory."

Any waves from this particular black hole merger have long past Earth, Miller said. But since galactic mergers are not unusual in the universe, black hole mergers are probably not either.

So there are probably gravitational waves passing through our solar system all the time, though scientists have yet to detect them.