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“Rogue” black holes out there, but fear not: astronomers

April 29, 2009
Courtesy Harvard-Smithsonian 
Center for Astrophysics
and World Science staff

It sounds like a sci-fi mov­ie plot: rogue black holes roam­ing our gal­axy, threat­en­ing to swal­low an­y­thing that gets too close. In fact, new cal­cula­t­ions sug­gest that hun­dreds of mas­sive black holes, left over from the gal­ax­y-build­ing days of the early uni­verse, may wan­der our Milky Way gal­axy.

Good news, how­ev­er: Earth is safe: say the re­search­ers who crunched the num­bers, Ryan O’Leary and Avi Loeb of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass. The clos­est rogue black hole should re­side thou­sands of light-years away, they ex­plain. As­tro­no­mers are ea­ger to find them, though, for the clues they would pro­vide to the gal­ax­y’s forma­t­ion.

“These black holes are relics of the Milky Way’s past,” said Loeb. “You could say that we are ar­chae­o­lo­gists.” 

Black holes are ex­treme­ly com­pact bod­ies whose gra­vity per­man­ently traps any ob­ject that wan­ders too close. Ac­cord­ing to the­o­ry, rogue black holes orig­i­nally lurked at the cen­ters of ti­ny ga­lax­ies that over bil­lions of years smashed to­geth­er to form full-sized ga­lax­ies like ours. Each time ga­lax­ies with cen­tral black holes col­lid­ed, their black holes merged to form a sin­gle, “re­lic” black hole. 

Dur­ing the merg­er, di­rec­tion­al emis­sion of rip­ples in space-time, known as gravita­t­ional radia­t­ion, would kick a black hole out­ward fast enough to es­cape its gal­axy, but not fast enough to leave the neigh­bor­hood com­plete­ly. As a re­sult, such black holes would still be around to­day in the out­er reaches of the Milky Way, a re­gion known as the ga­lac­tic 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, the re­search­ers said. 

The ob­jects 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 say they would have to look for more sub­tle clues to its ex­ist­ence and or­i­gin. For ex­am­ple, its light spec­trum would show that mul­ti­ple stars were pre­s­ent, mov­ing rap­idly to­geth­er, their paths in­flu­enced by the black hole’s gra­vity.

“The sur­round­ing star clus­ter acts much like a light­house that pin­points a dan­ger­ous reef,” said 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.

Lo­cat­ing the star clus­ter sign­posts may turn out to be rel­a­tively straight­for­ward, Loeb re­marked. “Un­til now, as­tro­no­mers were not search­ing for such a popula­t­ion of highly com­pact star clus­ters in the Milky Way’s ha­lo,” he noted. “Now that we know what to ex­pect, we can ex­am­ine ex­ist­ing sky sur­veys for this new class of ob­jects.” Loeb and O’Leary’s work is to be pub­lished in the re­search jour­nal Monthly No­tices of the Roy­al As­tronomical So­ci­e­ty.

* * *

It sounds like a sci-fi movie plot: rogue black holes roaming our galaxy, threatening to swallow anything that gets too close. In fact, new calculations suggest that hundreds of massive black holes, left over from the galaxy-building days of the early universe, may wander our Milky Way galaxy. Good news, however: Earth is safe: say the researchers who crunched the numbers, Ryan O’Leary and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. The closest rogue black hole should reside thousands of light-years away, they explain. Astronomers are eager to find them, though, for the clues they would provide to the galaxy’s formation. “These black holes are relics of the Milky Way’s past,” said Loeb. “You could say that we are archaeologists.” According to theory, rogue black holes originally lurked at the centers of tiny galaxies that over billions of years smashed together to form full-sized galaxies like ours. Each time galaxies with central black holes collided, their black holes merged to form a single, “relic” black hole. During the merger, directional emission of ripples in space-time, known as gravitational radiation, would kick a black hole 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, the researchers said. The objects 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 light spectrum would show that multiple stars were present, moving rapidly together, their paths influenced by the black hole’s gravity. “The surrounding star cluster acts much like a lighthouse that pinpoints a dangerous reef,” said 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 would provide new clues about the history of our galaxy. Locating the star cluster signposts may turn out to be relatively straightforward, Loeb remarked. “Until now, astronomers were not searching for such a population of highly compact star clusters in the Milky Way’s halo… Now that we know what to expect, we can examine existing sky surveys for this new class of objects.” Loeb and O’Leary’s work is to be published in the research journal Monthly Notices of the Royal Astronomical Society.