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Monster black holes, quietly cruising the cosmos?

May 30, 2007
Courtesy American Physical Society
and World Science staff

Two merg­ing black holes can gen­er­ate a re­coil so pow­er­ful, the merged hole shoots out of its host gal­axy at up to 4,000 km (2,500 miles) per sec­ond, ac­cord­ing to a new com­put­er sim­ula­t­ion.

Its cre­ators said the work shows for the first time that these vi­o­lent events, which fol­low merg­ers of ga­lax­ies con­tain­ing black holes, can to­tally eject the black holes. So these ti­ta­nic ob­jects may be cruis­ing through the un­iverse, vir­tu­ally in­vis­i­ble un­less they should crash in­to some­thing. 

But don’t wor­ry, as­tro­no­mers said. Most of the un­iverse by far is emp­ty space. The odds that a black hole will streak through our so­lar sys­tem are ti­ny.

Black holes are ex­tremely com­pact ob­jects that con­tain so much mat­ter crammed in­to so small a space that their gra­vity be­comes over­pow­ering and sucks in eve­ry­thing near­by, in­clud­ing light. De­spite their light-eating tal­ents, many black holes are as­sociat­ed with in­tense light emis­sions, be­cause the in­fall­ing ob­jects heat up and shine. But a black hole with noth­ing to feed on, called a “qui­es­cent” black hole, is dark.

Most lu­mi­nous ga­lax­ies are believed to con­tain a gi­ant, or supe­rmassive, black hole at their cen­ter. The sim­ula­t­ion, led by Manuela Cam­pan­elli at the Roch­es­ter In­sti­tute of Tech­nol­o­gy, N.Y., stud­ied the best con­di­tions for mer­gers to pro­duce re­coil speeds high enough to free a supe­rmassive black hole from its host gal­axy. 

The re­coil would re­sult when, up­on crash­ing, the black holes cre­ate an ex­ot­ic type of radia­t­ion called gravita­t­ional waves. In Cam­pan­elli’s sce­nar­i­o, two black holes ap­proach and start to or­bit each oth­er. To pro­duce to­tal ejection, they should have equal mass­es and spin as fast as pos­si­ble. They must be tilted with their ax­es of rota­t­ion ly­ing in the plane of their or­bit, and must spin in op­po­site di­rec­tions. 

They spir­al to­ward one anoth­er, and when they merge, the re­sulting ob­ject is kicked off pe­rpendicularly to the plane of or­bit. Some as­t­ro­phys­i­cists have ar­gued that such con­di­tions are rath­er un­like­ly; sci­en­tists said the prob­a­bil­ity of such a con­flu­ence of events re­mains a ques­tion for fu­ture re­search.

Past cal­cula­t­ions have found that black hole ejections may not be un­com­mon. But the ex­pelled black hole can easily fall back in­to the gal­axy due to con­tin­u­ing gravita­t­ional at­trac­tion be­tween the two, just as a can­non­ball shot to the sky re­turns to the ground.

A sec­ond new stu­dy, by Abra­ham Loeb of Har­vard Un­ivers­ity in Cam­bridge, Mass., ex­am­ined the pos­si­bil­ity of de­tect­ing a black hole if it is ex­pelled. If it’s sur­rounded by gas, he said, that gas will emit pow­er­ful light. Un­for­tu­nate­ly, by the time it leaves the gal­axy, it will likely ex­haust its gas supply and go dark.

None­the­less, one vis­i­ble ob­ject known as HE0450-2958, es­ti­mat­ed to lie more than three bil­lion light-years away, is the­o­rized by some to be an ejected supe­rmassive black hole. One of the re­search­ers who ad­vanced the pro­pos­al has said this black hole may be one of those that one day re­turns to its home gal­axy. It’s es­ti­mat­ed to have been mov­ing much more slowly on av­er­age than the fully-e­jected mono­liths that Cam­pan­elli stud­ied, en­hanc­ing the like­li­hood of an even­tu­al fall­back.

Loe­b’s and Cam­pan­elli’s stud­ies are to ap­pear in forth­com­ing is­sues of the re­search jour­nal Phys­i­cal Re­view Let­ters.

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Homepage image: artist's concept of a cross-section of an almost quiescent black hole.

Two merging black holes can generate a recoil so powerful, the merged hole shoots out of its host galaxy at up to 2,500 miles per second, according to a new computer simulation. Its creators said the work shows for the first time that the violent events, which follow mergers of galaxies containing black holes, can totally eject the black holes. So these gargantuan objects may be cruising through the universe, virtually invisible unless they should crash into something. But don’t worry, astronomers said. Most of the universe by far is empty space. The odds that a black hole will streak through our solar system are tiny. Black holes are extremely compact objects that contain so much matter crammed into so a small space that their gravity becomes overpowering and sucks in everything nearby, including light. Despite their light-eating talents, black holes are often visible, because the infalling objects heat up and shine. But a black hole with nothing to feed on, called a “quiescent” black hole, is dark. Most luminous galaxies contain a giant, or supermassive, black hole at their center. The simulation, led by Manuela Campanelli at the Rochester Institute of Technology, N.Y., studied the best conditions for producing recoil speeds high enough to free a supermassive black hole from its host galaxy. The recoil would result when, upon crashing, the black holes create an exotic type of radiation called gravitational waves. In Campanelli’s scenario, two black holes approach and start to orbit each other. To produce total ejection, they should have equal masses and spin as fast as possible. They must be tilted with their axes of rotation lying in the plane of their orbit, and must spin in opposite directions. They spiral toward one another, and when they merge, the resulting object is kicked off perpendicularly to the plane of orbit. Some astrophysicists have argued that such conditions are rather unlikely; scientists said the probability of such a confluence of events remains a question for future research. Past calculations have found that black hole ejections may not be uncommon. But the expelled black hole can easily fall back into the galaxy due to continuing gravitational attraction between the two, just as cannonball shot to the sky returns to Earth. A second new study, by Abraham Loeb of Harvard University in Cambridge, Mass., examined the possibility of detecting a black hole if it is expelled. If it’s surrounded by gas, he said, that gas will emit powerful light. Unfortunately, by the time it leaves the galaxy, it will likely exhaust its gas supply and go dark. Nonetheless, one visible object known as HE0450-2958, estimated to lie more than three billion light-years away, is theorized by some to be an ejected supermassive black hole. One of the researchers who advanced this proposal said this black hole may be one of those that one day returns to its home galaxy. It’s estimated to have been moving much more slowly on average than the fully-ejected monoliths that Campanelli studied, enhancing the likelihood of an eventual fallback. Loeb’s and Campanelli’s studies are to appear in forthcoming issues of the research journal Physical Review Letters.