Detailed Hubble images reveal a single supermassive black hole wandering away from its host galaxy's center where it belongs. The misplaced black hole is probably the result of a merger between two smaller black holes, but could also have been pushed by a jet of matter extending from the galaxy's core.

Nearly every galaxy has a supermassive black hole – millions to billions of times more massive than our sun – nestled in its center. Astronomers think galaxies frequently collide and merge to make bigger galaxies. When the galaxies merge, the theory goes, so do their black holes. Previous observations have caught such mergers in the act – but always when the black holes were thousands of light years apart, before they merged.

"This is the first time we have seen the merger after it has happened," said Eric Perlman of the Florida Institute of Technology at a press conference at the American Astronomical Society meeting in Miami on May 25. The results will also appear in a paper in an upcoming issue of Astrophysical Journal Letters.

Perlman and colleagues analyzed images of the largest local galaxy, M87, taken with the Hubble Space Telescope's highest-resolution camera, the Advanced Camera for Surveys. Black holes by nature are invisible, but the mass of gas and dust that heats up as it falls onto the guzzling giant, called an active galactic nucleus, can glow brightly and give away its location. The team located this bright nucleus, and found that it was 22 light-years away from the galaxy's center.

"The supermassive black hole is not where it is expected to be," said Daniel Batcheldor of the Florida Institute of Technology at the same press conference, but it's "a very slight, subtle offset."

The researchers came up with four possible explanations for the wayward black hole. First they thought it could be one of a pair of black holes spiraling in toward a merger. But the second black hole, which would have to be around the same size as the first, was nowhere to be found.

"It would be very very difficult for nature to mask such a large mass in that galaxy," Batcheldor said.

The researchers also dismissed the idea that the black hole was nudged aside by the combined gravitational tugs of other galaxies and globular clusters in M87's immediate neighborhood, the Virgo Cluster. All that mass would be enough to move the black hole just 0.3 light-years.

A more realistic possibility is the jet of material extending more than 5,000 light years from M87 could have pushed the black hole away from the center of the galaxy. The jet is massive enough to provide only a soft push on galactic scales, but it could be sufficient to move the black hole if the jet were 100 million years old or older. The jet would also have had to be much more massive in the past for this scenario to work, Batcheldor said.

"We don't rule this out as an offset mechanism because we do not know the history of accretion in M87 very well," he said. "It's very possible that in the past … there was more jet power that could have produced the offset."

The most likely solution is that the black hole is the product of a merger between two smaller black holes. Theory states that when two black holes merge, they emit gravitational waves that can give the resulting large black hole a "kick" of momentum, sending it flying through the galaxy. Because many galaxies are similar to M87, wandering black holes could be common in the universe.

"We expect there to be displaced black holes in the universe due to gravitational wave recoil," commented Julie Comerford of the University of California, Berkeley, who was not involved in the study. "This work shows that the black hole in M87 is a compelling candidate for a recoiling black hole."

Regardless of how M87's black hole left home, Batcheldor said, astronomers should rethink how black holes and galaxies normally fit together.

"It could well be that we need to shift the standard supermassive-black-hole paradigm that black holes are at the centers of galaxies, and revise it slightly to say that they're near the centers of galaxies," he said.

Image: NASA

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