Monster black holes, quietly cruising the cosmos?
Jun 01,2007 00:00 by Bend_Weekly_News_Sources

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 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 in falling 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 super massive, 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 super massive 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 super massive 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.





Courtesy American Physical Society and World Science staff