UCLA team finds evidence of massive black holes around Milky Way center

Jennifer Lauren Lee
Published: Friday, January 21, 2005

Twenty-six thousand light years away, at the center of our galaxy, a band of renegade black holes is congregating like a horde of bandits, flinging the hapless stars that cross its path into space, sending some directly into the maw of the massive black hole in the middle of the Milky Way, and possibly shaking the fabric of space-time itself.

Last Monday at a meeting of the American Astronomical Society in San Diego, Michael Muno, a UCLA postdoctoral Hubble Fellow, presented the first evidence of a massive conglomeration of tens of thousands of black holes surrounding the colossal black hole at the center of the Milky Way galaxy.

Mark Morris, co-author of Muno’s report and a UCLA astronomy professor, says the finding will help scientists learn how our galaxy, as well as others, evolved.

“Galaxy evolution is apparently tied to the evolution of the supermassive black hole at the center,” said Morris. “We don’t yet know all that much about how these black holes grow.”

Nearly 4 million times the mass of the sun, the supermassive black hole at the center of the galaxy grows by devouring nearby stars, including black holes, Morris said. He explained that the presence of so many black holes near the monstrous central black hole could play a vital role in how it feeds and grows.

“If there is a cluster of black holes there, that would alter the rate at which stars get eaten by that (central) black hole,” Morris said.

With their immense gravitational pull, black holes influence the orbits of stars in the neighborhood of the center of the galaxy, kicking them out of reach of the massive central black hole – or pushing them closer toward it, Morris said.

According to Muno, even light cannot escape their immense gravitational pull, forcing his team to infer their presence indirectly, by searching for “transient” X-ray sources – explosions of light bursting at frequencies a hundred times higher than the human eye can see, and lasting months at a time.

Muno’s team, which includes UCLA astronomy professor Andrea Ghez, collected its data using NASA’s Chandra X-ray Observatory, according to the Harvard University Chandra Web site.

“Chandra’s much more sensitive than any instrument we’ve had access to,” Muno said, with images 10 to 30 times sharper than those of other X-ray detectors.

These X-ray bursts are a sign that a super-dense object like a black hole is closely orbiting another star, Morris said.

In close contact with another star, Morris said, a black hole will begin to tear its lighter companion to pieces, sucking its matter into a hot, spiralling, disk-shaped vortex surrounding the black hole. He added that the disk of abducted material is highly unstable and can crash into the black hole suddenly, sending out its brief but potent bursts of energy as X-rays.

“Once in a while, these disks will undergo a total convulsion – cause, effectively, an explosion, (and) just start spewing out X-rays,” Morris said.

Using this method, Muno and his team identified seven sources of transient X-ray bursts within 75 light years of the supermassive central black hole, a much higher concentration of black holes than scientists had imagined to exist.

As the densely packed remnants of dead stars, black holes should be distributed as evenly throughout the galaxy as other stars are, Morris said.

But the X-ray bursts, betraying the presence of black holes and other massive objects, imply that the concentration of black holes in the central few light years of the galaxy is higher than it should be, Morris said. According to Morris, this suggests that, slowly but surely, the black holes are migrating inward.

“We didn’t expect to see black holes (so) close to the central black hole,” Muno said. “Given the number of stars ... (there was) only a 20 percent chance that one of these would be so close to the galactic center.”

Muno said this supports Morris’ 1993 prediction that black holes should congregate at the center of the galaxy, due to a process by which heavier objects – such as black holes – sink to the center of the galaxy like rocks tumbling down a well.

More often than not, Morris said, when a pair of stars encounter one another, the heavier star gives some of its energy to the lighter one, flinging its lighter partner into space. The heavier star sinks slowly toward the supermassive black hole over billions of years until it, too, meets its fate by merging with the black hole.

“They’re at the bottom of the well. They can’t get out – it’s inevitable. ... There’s no magic fairy that’s going to pull them out of that pit,” Morris said.

Collisions of such massive objects as black holes may also cause ripples in the fabric of space-time, or so-called “gravity waves,” which – if they are detected – could be the first confirmation of a very important prediction of Einstein’s theory of gravity, Muno said.

Muno said his team plans to continue to use Chandra to study these “black holes” in the center of the galaxy, to confirm that the dense objects setting off these X-ray bursts really are black holes.

“We can’t categorically rule out neutron stars,” Morris said, explaining that these stars, although not much more massive than our sun and not nearly as massive as black holes, could also produce X-ray bursts like those Chandra has detected. But, he adds, there are ways for scientists to distinguish between the two, with further research.

“(With Chandra), we’re seeing things that no one had ever seen before ... and confirming things that people did expect but never thought they could see,” Muno said.