Published online 1 May 2008 | Nature | doi:10.1038/news.2008.792

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Fast-moving black holes leave behind hole-less galaxies.

Are some super-massive black holes screaming through the Universe at speed? NASA

A black hole 600 million times the mass of the Sun has been kicked out of the centre of a distant galaxy, according to research from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

The first-of-its-kind observation, if confirmed, would be a long-awaited validation for theorists. It has long been predicted that when massive black holes merge with each other they can cast off so much energy that they recoil with enough speed to escape their resident galaxy. But an earlier search for evidence of such runaway massive black holes turned up disappointingly empty (see Static holes defy theory).

If such black holes are running wild in intergalactic space, then some galaxies should no longer have black holes at their centres. That would raise many questions for astronomers, who have come to believe that black holes are crucial in controlling the growth of galaxies.

Stefanie Komossa, an astrophysicist at Max Planck, says her team got lucky in spotting the black hole among thousands of interesting candidates in the astronomical database produced by the Sloan Digital Sky Survey. "It instantly stuck out as something very special and unusual," says Komossa, lead author of the study, which will be published on 10 May in The Astrophysical Journal1. They think the escapee is moving at 2,650 kilometres per second.

Super massive

Black holes, objects with such strong gravitational pull that light can’t escape, often form when ordinary stars collapse. But these pale in size and mass to the 'super-massive' black holes (SMBHs) that lie at the centre of most galaxies and that grow as they devour stars.

They can also grow when two of them collide during the merger of two galaxies. For decades, general relativity theorists have tried to simulate the complicated 'death spiral' of merging SMBHs. The final moments of the merger carry huge amounts of momentum, which should be able to exert a significant kick on the merged black hole.

Early simulations showed maximum kick velocities of a few hundred kilometres per second — fast enough to eject black holes from small dwarf galaxies. But in the past few years, newer models, which include the spins of the SMBHs, show that the black holes should be able to recoil at thousands of kilometres per second — fast enough to escape the tug of galaxies of any size.

The report from Komossa’s team is the first observation of this — an "enormous validation" for the theorists, says Scott Hughes, an astrophysicist at the Massachusetts Institute of Technology in Cambridge. "The smoking gun analogy really works," he says. "This is the recoiling gun."

Well spotted

Komossa looked for the kicked black hole by examining wavelengths of light emitted by gases that swirl around black holes just before being swallowed. Theory supports the idea that a kicked black hole carries some gas with it and leaves some behind in the galaxy. Komossa found evidence for two separate portions of such gas, and measured the difference in speed between them, producing a likely candidate for a fast-moving black hole.

She says that the kick probably happened in the past million years, because otherwise, the recoiling black hole would run out of glowing material to eat up. In that time, the black hole may have travelled several thousand light years, she says.

Komossa hopes to corroborate the discovery with a follow-up observation by the Hubble Space Telescope, which has high enough resolution to spot a spatial difference between the galaxy and its black hole.

Something to see here

The result does more than just please the modellers. Astrophysicists have been planning missions such as the Laser Interferometer Space Antenna (LISA), which would measure the subtle ripples in space caused by faint gravity waves. But they don’t know how common gravity-wave producing events, such as black-hole mergers, really are. Komossa’s result gives LISA’s proponents some assurance that there will something to detect.

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Also, SMBHs are intimately tied to the evolution of galaxies. They release jets of energy that can shock gas nebulae, turning on the gravitational collapse of gas that leads to star formation. Astronomers have noticed a reliable correlation: the bigger the SMBH, the bigger the bulge of stars in the core of the galaxy. If galaxies without black holes at their centre turn out to be relatively common, then explaining galaxy formation and evolution could become problematic.

Astronomers are excited by the result, but are eager to see some corroboration. Erin Bonning, a postdoctoral researcher in astrophysics at Yale University in New Haven, Connecticut, last year conducted the study that turned up no signs of such fast-moving holes, and says the computer algorithm she used missed this particular one. “It is making us wonder what else we may have missed,” she says. She says that Komossa’s data and analysis seem sound. “It’s the most convincing thing I’ve seen to date."