Grav­i­ta­tional waves push black hole out of gal­axy, re­port says

The Washington Post Sunday - - POLITICS & THE NATION - BY SARAH KA­PLAN sarah.ka­plan@wash­

Black holes are the big bul­lies of space. They are so mas­sive that their grav­ity does not let any light es­cape. The biggest black holes — called “su­per­mas­sive” — weigh as much as a bil­lion suns. Loom­ing at the cen­ter of seem­ingly ev­ery gal­axy, in­clud­ing the Milky Way, they con­trol the for­ma­tion of stars and can de­form the fab­ric of space-time it­self. It takes a lot to push a black hole around.

But 8 bil­lion light-years from Earth, in a gal­axy called 3C 186, as­tronomers have dis­cov­ered a su­per­mas­sive black hole that got kicked off its throne. It is now rock­et­ing through space at a speed of al­most 5 mil­lion miles per hour.

There is one thing that could un­seat a su­per­mas­sive black hole in this man­ner, the re­searchers say: grav­i­ta­tional waves.

First pre­dicted by Al­bert Ein­stein more than 100 years ago, grav­i­ta­tional waves are rip­ples in space-time caused by the uni­verse’s most cat­a­clysmic events — just as con­cen­tric cir­cles form on the sur­face of a pond after you toss in a heavy rock. Last year, re­searchers at the Laser In­ter­fer­om­e­ter Grav­i­ta­tional-Wave Ob­ser­va­tory (LIGO) in Pasadena, Calif., showed that this phe­nom­e­non ex­ists when they de­tected grav­i­ta­tional waves pro­duced by the merger of two black holes.

In a pa­per that will be pub­lished this week in the jour­nal Astron­omy & Astro­physics, Marco Chi­aberge and his col­leagues say that the weird be­hav­ior of the black hole in gal­axy 3C 186 is likely the re­sult of grav­i­ta­tional waves from an­other pair of col­lid­ing black holes.

The rov­ing black hole was de­tected in an im­age taken by NASA’s Hub­ble Space Te­le­scope. The fuzzy splotch that was gal­axy 3C 186 con­tained an in­cred­i­bly bright spot, a quasar. This wasn’t un­usual — a quasar is the nu­cleus of a gal­axy, and it is bright be­cause of the disk of gas that sur­rounds the black hole at its cen­ter.

What caught Chi­aberge’s eye was the quasar’s lo­ca­tion, 35,000 light-years from the cen­ter of its gal­axy.

“I thought we were see­ing some­thing very pe­cu­liar,” he said in a NASA news re­lease.

Chi­aberge, who works at the Space Te­le­scope Sci­ence In­sti­tute (STScI) and Johns Hop­kins Uni­ver­sity, asked fel­low as­tronomers for their ob­ser­va­tions from a range of other in­stru­ments, in­clud­ing the Chan­dra space ob­ser­va­tory and the Sloan Dig­i­tal Sky Sur­vey’s te­le­scope in New Mex­ico. The for­mer mea­sures X-rays; the lat­ter spe­cial­izes in de­tect­ing red­shift, the stretch­ing of light that is de­tected as some­thing trav­els through space.

Their ob­ser­va­tions con­firmed the Hub­ble find­ing. They also helped pin down the black hole’s mass (equal to that of a bil­lion suns) and the speed at which the gas around it was trav­el­ing (4.7 mil­lion mph).

Mean­while, the Hub­ble im­age of­fered a clue about what dis­lodged the black hole from its gal­axy’s cen­ter. The host gal­axy bore faint, arc-shaped fea­tures called tidal tales, which are pro­duced by the grav­i­ta­tional tug-ofwar that takes place when two gal­ax­ies col­lide. This sug­gested that gal­axy 3C 186 had re­cently merged with an­other sys­tem, and per­haps their black holes merged, too.

What hap­pened next, sci­en­tists can only the­o­rize. Chi­aberge and his col­leagues sug­gest that as the gal­ax­ies col­lided, their black holes be­gan to cir­cle each other, fling­ing out grav­ity waves “like wa­ter from a lawn sprin­kler,” as NASA de­scribed it. If the black holes had un­equal masses and spin rates, they might have sent more grav­i­ta­tional waves in one di­rec­tion than the other. When the col­li­sion was com­plete, the newly merged black hole would have then re­coiled from the strong­est grav­i­ta­tional waves, shoot­ing off in the op­po­site di­rec­tion.

“This asym­me­try de­pends on prop­er­ties such as the mass and the rel­a­tive ori­en­ta­tion of the [black holes’] ro­ta­tion axes be­fore the merger,” Colin Nor­man of STScI and Johns Hop­kins Uni­ver­sity, a co-au­thor on the pa­per, said in the NASA news re­lease. “That’s why th­ese ob­jects are so rare.”

There is an al­ter­na­tive ex­pla­na­tion for the rov­ing black hole, the re­searchers said. It is pos­si­ble that the quasar only ap­pears to be lo­cated in gal­axy 3C 186, but is be­hind it — ex­plain­ing why the gal­axy’s nu­cleus seems to be of­f­cen­ter.

But if that is the case, the sci­en­tists say, re­searchers should have de­tected the quasar’s ac­tual host gal­axy — and they haven’t yet. If Chi­aberge’s in­ter­pre­ta­tion is cor­rect, it can help as­tronomers un­der­stand what hap­pens in a black hole merger.

Even with­out know­ing the source of the be­hav­ior, the sci­en­tists have drawn some pretty in­cred­i­ble con­clu­sions about it. They es­ti­mate that the en­ergy re­quired to jet­ti­son a black hole like the one in 3C 186 would be equiv­a­lent to 100 mil­lion su­per­novas. Now the black hole is mov­ing so fast it could cover the dis­tance be­tween the Earth and the Moon in a mere 3 min­utes. In about 20 mil­lion years, the as­tronomers pre­dict, it will es­cape its gal­axy and roam alone through the uni­verse for­ever.

What­ever is go­ing on with this bizarre black hole, it has cer­tainly had a wild ride.


The strange be­hav­ior of the black hole in gal­axy 3C 186 is likely the re­sult of grav­i­ta­tional waves from an­other pair of col­lid­ing black holes, re­searchers say. The bright spot in the im­age’s cen­ter is a quasar.

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