Hundreds of "Rogue" Black Holes May Roam Milky Way

Anne Minard
for National Geographic News
January 10, 2008
Newly merged black holes might be so jarred by the experience that they go "rogue," careening into space on unexpected trajectories.

In fact, hundreds of these rogue black holes could exist in the Milky Way galaxy alone, according to a new study.

The research is just one of several new findings on black holes presented Wednesday at a meeting of the American Astronomical Society in Austin, Texas.

Together, the observations are helping unravel some mysteries of the superdense objects—while simultaneously offering perplexing new puzzles.

Scientists at the presentation, for instance, offered up new explanations about odd x-ray sources coming from black holes.

They also found hints that dark matter may play an important role in the hot disks that form around companion-consuming black holes.

Black Hole Billiards

Astronomers have long known about rogue black holes.

But Kelly Holley-Bockelmann, of Tennessee's Vanderbilt University, and her colleagues were the first to show that the objects could arise from violent mergers.

Using a computer model, Holley-Bockelmann found that two combining black holes rotating at different speeds or of different sizes give the newly merged black hole a big kick.

This sends the object hurtling in an arbitrary direction at velocities as high as 2,485 miles (4,000 kilometers) per second.

"This is much higher than anyone predicted," Holley-Bockelmann said. "Even the average kick velocity of 200 kilometers [124 miles] per second is extremely high."

Of vital importance is that such merged objects are predicted to be intermediate-size—about as massive as a few thousand suns—a class of black holes that remains theoretical.

Until now, scientists thought drifting black holes could only be supermassive—on the order of hundreds of thousands of suns—or the smallest class, just a few hundred times the mass of the sun.

Roaming Rogues

The finding may have important implications for our galaxy.

Just about any black hole merger would fling the new remnant out of the globular cluster where it formed, Holley-Bockelmann said—at speeds up to 9 million miles (14 million kilometers) an hour.

Globular clusters are ancient star clusters that have about a million stars in them and could also have intermediate black holes at their centers.

If the roughly 200 globular clusters in the Milky Way have indeed spawned intermediate-size black holes, this means that hundreds of such rogue objects are probably wandering invisibly around the Milky Way.

(Related: "1,000 Black Holes Revealed in New Sky Survey" [March 13, 2007].)

"People ask me, Is this dangerous? Do I need to build a black hole shelter?" Holley-Bockelmann said. "The answer is, No, you don't."

She said the only possible danger is that a meandering black hole would plow through the Oort cloud—a cloud of comets thought to exist at the outer edge of our solar system—and kick the objects on a path toward Earth.

"But that's a 1 in 10 quadrillion per year probability," she said.

X-Ray Vision

Emerging science is also shedding light on the bizarre X-ray features of black holes.

The nearby galaxy Centaurus A, for example, contains giant X-ray jets emanating from its center, said Gregory Sivakoff, a postdoctoral researcher in astronomy at Ohio State University.

The jets, a mark of energy-emitting objects known as active galaxies, extend for 13,000 light-years in different directions. (Related: "'Death Star' Galaxy Found Blasting Smaller Neighbor [December 17, 2007].)

But Centaurus A recently revealed a lower-key source of X-rays.

Sivakoff believes it's coming from a binary star system where one star collapsed to form a black hole—and is now devouring its companion.

The find, he said, "is actually pointing to a puzzle rather than a solution."

This type of star system is supposed to be rare, but it's the second such system discovered in Centaurus A.

"When we look at other galaxies like Centaurus A, we don't see these bright, transient X-ray binaries," Sivakoff said. "But now we've found two such objects in Centaurus A, and the implication is that we may not understand these objects as well as we thought we did."

Hungry and Dark

New observations of a binary star system, meanwhile, are forcing a reassessment of the disks of material that form around a wide variety of astronomical objects.

Steve Howell, an astronomer at the National Optical Astronomy Observatory in Tucson, Arizona, and his colleagues used space- and ground-based telescopes to peer at the star system WZ Sagittae.

Known as an interacting binary star, WZ Sagittae contains a white dwarf—a star about the size of Earth but with the mass of the sun—and a larger, less dense, cooler companion star.

The white dwarf usually rips material off its companion and pulls it inward to form a disk, known as an accretion disk.

Howell and his team noticed the disk is larger in infrared than it appeared in optical studies.

"Our observations strongly imply the presence of dark matter in these structures, which are ubiquitous throughout the universe," Howell said.

"This discovery suggests that our current model for accretion disks of all kinds is wrong," added team member Donald Hoard, of NASA's Spitzer Science Center.

"We will need to rethink and recast these models for accretion disks, not only in interacting binary stars but also in distant, highly luminous active galaxies."

New Questions

AAS President Craig Wheeler, author of the author of the 2007 book Cosmic Catastrophes: Supernovae, Gamma-ray Bursts, and Adventures in Hyperspace, was an independent commentator at the black holes briefing.

He called black holes "amazingly exciting things," worthy of their center-stage presence in the scientific and public eye.

"Black holes are so well known now that they are almost a cliché," he said. "But they are still large in the public perception ... and still an immensely important and challenging topic for astrophysicists."

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