Milky Way: Black Holes, Thousands of Them, Discovered

This is an artist's impression showing the peanut shaped structure in the central bulge.		    Credit ESO  NASA  JPL-Caltech  M. Kornmesser  R. Hurt			  Close

A new study supports the prevailing theory of where some of the universe's most mysterious inhabitants - black holes - reside. "There might be some non-black-hole objects such as unusual stars lumped in with the black hole sample". So, too, do astrophysical exotica such as neutron stars and white dwarfs-the remnants left by normal stars when they die. Over the last 12 years, Chandra has observed Sagittarius A* many times - adding up to a huge total of two weeks of exposure time. These could follow the motion of stars very close to the unseen object as they orbited around it. But if you took the equivalent space around Earth there would be zero black holes, not thousands, Hailey said.

"This is just kind of astonishing that you could have a prediction for such a large number of objects and not find any evidence for them", Hailey says. The team speculates these must be the first observational signs of the long-theorized "cusp".

If you think about it, galaxies are pretty unusual.

Scientists have suggested for decades that lots of stellar black holes are circling in the center of galaxies, including ours.

The findings are reported in the latest issue of Nature journal.

The biggest black holes have masses ranging from millions to billions of times that of the Sun. "There hasn't ever been much controversy about this idea, because it's just an inevitable outcome of simple Newtonian dynamics", Morris says.

The problem with this theory is that, until now, it's been exceptionally hard to prove.

In the new study, Charles Hailey, an astrophysicist at Columbia University, and his colleagues scrutinized the past dozen years of data gathered by the Chandra X-ray Observatory, an orbiting craft whose instruments are created to detect high-energy radiation emitted by the immensely hot material surrounding exploded stars and near black holes.

They then analyzed the spatial distribution and properties of the identified binary systems and concluded from their observations that there must be anywhere from 300 to 500 such binaries and roughly 10,000 isolated black holes in the area surrounding Sagittarius A*. Furthermore, all black holes in the nearby vicinity of Sagittarius A* are held close by its massive gravitational pull.

Hailey said the new findings could significantly advance research into one of the most exciting fields of astronomy right now: gravitational waves.

A search for the X-ray signatures of low-mass black hole binaries in the Chandra data turned up 12 within three light-years of Sgr A*.

"I think this is a really intriguing result", says Fiona Harrison, an astrophysicist at Caltech. "A discovery like this will always have consequences that we can not presently predict", he says.

"It's an obvious way to want to look for black holes", Hailey said, "but the galactic center is so far away from Earth that those bursts are only strong and bright enough to see about once every 100 to 1,000 years".

"But black holes with a stellar companion make X-rays". Morris says. "Hundreds of papers have been written already speculating about this".

Black holes are generally "pretty impossible" to see, according to the physicist.

They found a dozen of these black hole binaries, and because it's rare for a black hole to grab a star, the scientists can infer that many others are out there that didn't grab a star and are thus invisible. Since the ripples are emitted whenever black holes collide, or fall into the center of the galaxy, estimating the population of these black holes might help refine the estimates of the number of gravitational wave events.

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