Rogue Black Holes May Roam the Milky Way

April 29, 2009

This artist's conception shows a rogue black hole floating near a globular star cluster on the outskirts of the Milky Way. New calculations by Ryan O'Leary and Avi Loeb suggest that hundreds of massive black holes, left over from the galaxy-building days of the early universe, may wander the Milky Way. Fortunately, the closest rogue black hole should reside thousands of light-years from Earth. Credit: David A. Aguilar (CfA)
( -- It sounds like the plot of a sci-fi movie: rogue black holes roaming our galaxy, threatening to swallow anything that gets too close. In fact, new calculations by Ryan O'Leary and Avi Loeb (Harvard-Smithsonian Center for Astrophysics) suggest that hundreds of massive black holes, left over from the galaxy-building days of the early universe, may wander the Milky Way.

Good news, however: Earth is safe. The closest rogue black hole should reside thousands of light-years away. Astronomers are eager to locate them, though, for the clues they will provide to the formation of the .

"These are relics of the Milky Way's past," said Loeb. "You could say that we are archaeologists studying those relics to learn about our galaxy's history and the formation history of black holes in the ."

According to theory, rogue black holes originally lurked at the centers of tiny, low-mass galaxies. Over billions of years, those dwarf galaxies smashed together to form full-sized galaxies like the Milky Way.

Each time two proto-galaxies with central black holes collided, their black holes merged to form a single, "relic" black hole. During the merger, directional emission of gravitational radiation would cause the black hole to recoil. A typical kick would send the black hole speeding outward fast enough to escape its host dwarf galaxy, but not fast enough to leave the galactic neighborhood completely. As a result, such black holes would still be around today in the outer reaches of the Milky Way halo.

Hundreds of rogue black holes should be traveling the Milky Way's outskirts, each containing the mass of 1,000 to 100,000 suns. They would be difficult to spot on their own because a black hole is visible only when it is swallowing, or accreting, matter.

One telltale sign could mark a rogue black hole: a surrounding cluster of stars yanked from the dwarf galaxy when the black hole escaped. Only the stars closest to the black hole would be tugged along, so the cluster would be very compact.

Due to the cluster's small size on the sky, appearing to be a single star, astronomers would have to look for more subtle clues to its existence and origin. For example, its spectrum would show that multiple stars were present, together producing broad spectral lines. The stars in the cluster would be moving rapidly, their paths influenced by the gravity of the black hole.

"The surrounding star cluster acts much like a lighthouse that pinpoints a dangerous reef," explained O'Leary. "Without the shining stars to guide our way, the black holes would be all but impossible to find."

The number of rogue black holes in our galaxy depends on how many of the proto-galactic building blocks contained black holes at their cores, and how those proto-galaxies merged to form the Milky Way. Finding and studying them will provide new clues about the history of our galaxy.

Locating the star cluster signposts may turn out to be relatively straightforward.

"Until now, astronomers were not searching for such a population of highly compact star clusters in the Milky Way's halo," said Loeb. "Now that we know what to expect, we can examine existing sky surveys for this new class of objects."

Loeb and O'Leary's journal paper will be published in the Monthly Notices of the Royal Astronomical Society and is available online at .

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4 / 5 (2) Apr 29, 2009

I wonder if we'll ever get a follow up regarding their observations.
5 / 5 (4) Apr 29, 2009
I've always been a bit of a "satirical alarmist" about the senario of a rogue star or rogue BH wandering through our solar system... and how quickly things would end here if the Earth's orbit was significantly disturbed. I often tell people half-jokingly, "We gotta get off this planet!"

I recall a previous article (I think it was also in PhysOrg) about NASA or ESA tracking a "rogue star" that was screaming through our galaxy at enormous speed relative to the nearby stars it was passing. I'm also an avid follower of the Einstein@Home project for gravity wave detection from BH and neutron stars.

I still wonder if a single BH or neutron star moving at large relative velocity through the galaxy would leave a detectable "wake" in the surrounding gravity field, independant of any gravity waves it may produce due to it's own rotation, and afford us a way to detect these things.
4.3 / 5 (6) Apr 29, 2009
I wonder what the numerical density of black holes would have to be in order to account for the 'missing matter' of the universe .. A BH here, another 100 there, in a trillion galaxies here, a trillion-trillion there .. it might add up to somethin!

5 / 5 (3) Apr 29, 2009
I'm with tkjtkj regarding black holes and "dark matter" in wondering "Could there be enough of them to make up the difference?"
4.3 / 5 (3) Apr 29, 2009
Could this population account for some of the weird 'density curve' data that MOND 'modified Newtonian dynamics' was proposed to fix ??
4 / 5 (3) Apr 29, 2009
Surely a BH is no more sucky than any other object of equal mass? I don't think I'd care too much whether I was anihilated by a BH or some red giant passing our way. The story should have really focussed on the fact that any matter consumed by a black hole is locked away, and no longer contributes to the visible part of the galaxy. If there are many of them, what proportion of the total mass of the galaxy do they represent?
5 / 5 (1) Apr 30, 2009
Yup, and this wonderful new theory will explain the dark mass and energy very well.

Still, it's amazing what people can come up when they think about black hole. But apart from creativity, there's still too much of a discussion whether we have observed a black hole at all, so although fun, there's not much point of discussion probable dark objects flying around our galaxy.
not rated yet Apr 30, 2009
It's been a while since I followed this kind of optics, but shouldn't gravitational lensing catch these objects?
not rated yet Apr 30, 2009
It's been a while since I followed this kind of optics, but shouldn't gravitational lensing catch these objects?
Ideallistically, I would think it's possible, but I'm no expert.

Realistically, I imagine it would be resource intensive (or else take incredible luck) to find one this way.

Simplistically, I would think you'd need a method of baselining all the background images of the surrounding sky and then a system to monitor the sky in all (or many, or a few?) directions looking for visible changes specifically due to lensing.

That's why I was thinking first about a method of direct detection of the BH itself from gravity waves (via LIGO or LISA, or similar).
not rated yet Apr 30, 2009
Assuming we'll ever detect any gravitational waves.

They already have a common sense approach... Look for clusters of stars orbiting a non-luminous central point which itself is orbiting the galactic core.
not rated yet May 01, 2009
Assuming we'll ever detect any gravitational waves.
We better! I didn't burn up a hard drive crunching for Einstein@Home for nothing (or did I?)

They already have a common sense approach... Look for clusters of stars orbiting a non-luminous central point which itself is orbiting the galactic core.
Good point.

If the rogue BH was inside our galaxy rather than outside (closer to Earth and with closer objects behind it), how much more (or less) pronounced could the visible lensing effect be?
not rated yet May 05, 2009
We could detect them by means of gravitational lensing, but the mass is very small in comparison to a galaxy that is normally used for lensing. So in the most idealistic case, you can use it at very large distances from and to the lense and I think the distance to the lense is not big enough.
So I agree with barkster.
So the idea is great. Very good method although is real life other methods are preferred because they are easier and have better precision

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