"Long before it's in the papers"
January 27, 2015

RETURN TO THE WORLD SCIENCE HOME PAGE


Study proposes violent past for Milky Way

March 6, 2013
Courtesy Vanderbilt University
and World Science staff

These days the co­re of our Milky Way gal­axy is pret­ty qui­et, cos­mic­ally speak­ing. The ga­lac­tic black hole at the cen­ter is a sleep­ing gi­ant. Stars peace­fully cir­cle it.

But grow­ing ev­i­dence shows that sev­er­al mil­lion years ago the ga­lac­tic cen­ter was the site of vi­o­lent ce­les­tial fire­works, as­tro­no­mers say. They pro­pose that an an­cient crash and merg­er be­tween that giant black hole, and a mid-sized black hole in a neigh­bor­ing “satel­lite gal­axy,” could ex­plain sev­er­al odd re­cent ob­serva­t­ions.

An artist's il­lus­tra­tion of a sat­el­lite gal­axy fall­ing in­to the sub­mas­sive black hole at the cen­ter of the Milky Way gal­axy. (Cred­it: Ju­lie Turn­er, Van­der­bilt U.)


K­elly Holley-Bockel­mann of Van­der­bilt Uni­vers­ity in Nash­ville, Tenn. and Ta­mara Bog­danovic at the Geor­gia In­sti­tute of Tech­nol­o­gy pro­pose the sce­nar­i­o in the March 6 is­sue of the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.

In Jan­u­ary 2010, “Ta­mara and I had just at­tended an as­tron­o­my con­fer­ence in As­pen, Col­o­rado, where sev­er­al of these new ob­serva­t­ions were an­nounced,” said Holley-Bockel­mann. “A snow storm had closed the air­port. We de­cid­ed to rent a car to drive to Den­ver. As we drove through the storm, we pieced to­geth­er the clues from the con­fer­ence and real­ized that a sin­gle cat­a­stroph­ic even­t—the col­li­sion be­tween two black holes about 10 mil­lion years ago—could ex­plain all the new ev­i­dence.”

The most dra­mat­ic clue is “Fer­mi bub­bles,” she ex­plained. These are gi­ant, round pro­jec­tions of high-energy radia­t­ion ex­tend­ing some 30,000 light years above and be­low the Milky Way cen­ter. A light year is the dis­tance light trav­els in a year. The bub­bles are be­lieved to be caused by par­t­i­cles mov­ing at nearly light speed. Were the bub­bles vis­i­ble to the un­aided eye, they would fill about half of the night sky, but they’re not be­cause they give off only X-ray and gamma-ray light.

An­oth­er char­ac­ter­is­tic of the ga­lac­tic cen­ter is that it con­tains the three great­est clus­ters of young stars in the gal­axy, said Holley-Bockel­mann. These so-called Cen­tral, Arch­es and Quin­tu­plet clus­ters each con­tain hun­dreds of young, hot and huge stars. These typ­ic­ally burn out in “on­ly” a few mil­lion years be­cause of their ex­treme bright­ness, mean­ing there has been a rel­a­tively re­cent burst of star forma­t­ion, ac­cord­ing to the as­tro­no­mers.

A third piece of ev­i­dence is the su­per­mas­sive black hole that dom­i­nates the cen­ter of the Milky Way it­self, they added. A black hole is an ob­ject so heavy and com­pact that its gra­vity over­whelms any ob­jects near­by; these even­tu­ally fall in­to the black hole. The one at the heart of the Milky Way is es­ti­mat­ed to weigh the equiv­a­lent of about four mil­lion suns and to be about nine times as wide as the sun. 

Such an ob­ject pro­duces in­tense gravita­t­ional tides. So as­tro­no­mers were sur­prised to find a num­ber of clumps of bright new stars clos­er than three light years from the black hole’s maw. It would­n’t be that sur­pris­ing if the stars were be­ing sucked in­to the black hole, but they show eve­ry sign of hav­ing formed in place, the as­tro­no­mers said. For this to hap­pen, the clouds of dust and gas that they formed from must have been thou­sands of times thicker than nor­mally ex­pected for such clouds.

While there is an ex­cess of young hot stars in the ga­lac­tic co­re, there is al­so a sur­pris­ing dearth of old­er stars, the re­search­ers added. The­o­ret­i­cal mod­els pre­dict that there should be more old stars clos­er to the black hole, but there are very few with­in sev­er­al light years of the sleep­ing gi­ant.

Holley-Bockelmann and col­leagues came up with a sce­nario that they said fits these ob­serva­t­ions and makes some test­a­ble pre­dictions. The sto­ry be­gins about 13 bil­lion years ago, when one of the smaller sat­el­lite ga­lax­ies or­bit­ing the Milky Way be­gan drift­ing in­ward to­ward the co­re. An­oth­er re­cent study pro­poses this may have hap­pened doz­ens of times in the Milky Way’s life­time. As the sat­el­lite gal­ax­y—a col­lec­tion of stars and gas with a mid-sized black hole weigh­ing about 10,000 sun­s—spi­raled in­ward, most of its ma­te­ri­al was grad­u­ally stripped away, fi­nally leav­ing the black hole and a hand­ful of its sat­el­lite stars.

Some 10 mil­lion years ago, the sat­el­lite gal­ax­y’s stripped-down co­re would have fi­nally reached the ga­lac­tic cen­ter. The smaller black hole would then have spiraled in­ward to the the Milky Way’s black hole for sev­er­al mil­lion years be­fore be­ing final­ly en­gulfed. During this time, it would have churned up dust and gas and pushed enough ma­te­ri­al in­to the ga­lac­tic black hole to pro­duce the Fer­mi bub­bles, the re­search­ers claim. Mean­while, vi­o­lent gravita­t­ional tides could easily have com­pressed the mo­lec­u­lar clouds in the core to the ex­treme dens­i­ties re­quired to ex­plain all those young stars.

The vig­or­ous churn­ing would al­so have swept out the ex­ist­ing stars from the ar­ea sur­round­ing the mas­sive cen­tral black hole, they argue. In fact, the new mod­el pre­dicts that the black holes’ merg­er dance should have flung a large num­ber of the mis­sing old stars out in­to the gal­axy at “hy­per ve­lo­ci­ties,” ex­plaining the ab­sence of old stars in the ar­ea.

“The gravita­t­ional pull of the sat­el­lite gal­ax­y’s black hole could have carved nearly 1,000 stars out of the ga­lac­tic cen­ter,” said Bog­danovic. “Those stars should still be rac­ing through space, about 10,000 light years away from their orig­i­nal or­bits.” If the the­o­ry works, these should be iden­ti­fiable with large star sur­veys like the Sloan Dig­it­al Sky Sur­vey, an on­go­ing proj­ect, she added.


* * *

Send us a comment on this story, or send it to a friend









 

Sign up for
e-newsletter
   
 
subscribe
 
cancel

On Home Page         

LATEST

  • St­ar found to have lit­tle plan­ets over twice as old as our own

  • “Kind­ness curricu­lum” may bo­ost suc­cess in pre­schoolers

EXCLUSIVES

  • Smart­er mice with a “hum­anized” gene?

  • Was black­mail essen­tial for marr­iage to evolve?

  • Plu­to has even cold­er “twin” of sim­ilar size, studies find

  • Could simple an­ger have taught people to coop­erate?

MORE NEWS

  • F­rog said to de­scribe its home through song

  • Even r­ats will lend a help­ing paw: study

  • D­rug may undo aging-assoc­iated brain changes in ani­mals

These days the core of our Milky Way galaxy is pretty quiet, cosmically speaking. The galactic black hole at the center is a sleeping giant. Stars are peacefully circling. But growing evidence shows that several million years ago the galactic center was the site of violent celestial fireworks, astronomers say. They propose that an ancient crash and merger between the galactic black hole, and a mid-sized black hole in a neighboring “satellite galaxy,” could explain several intruiging recent observations. Kelly Holley-Bockelmann of Vanderbilt University in Nashville, Tenn. and Tamara Bogdanovic at the Georgia Institute of Technology propose the scenario in the March 6 issue of the journal Monthly Notices of the Royal Astronomical Society. In January 2010, “Tamara and I had just attended an astronomy conference in Aspen, Colorado, where several of these new observations were announced,” said Holley-Bockelmann. “A snow storm had closed the airport. We decided to rent a car to drive to Denver. As we drove through the storm, we pieced together the clues from the conference and realized that a single catastrophic event—the collision between two black holes about 10 million years ago—could explain all the new evidence.” The most dramatic clue is “Fermi bubbles,” she explained. These are giant, round projections of high-energy radiation extending some 30,000 light years above and below the Milky Way center. A light-year is the distance light travels in a year. The bubbles are believed to be caused by particles moving at nearly light speed. Were the bubbles visible to the unaided eye, they would fill about half of the night sky, but they’re not because they give off only X-ray and gamma-ray light. The finding was reported by astronomers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Another characteristic of the galactic center is that it contains the three greatest clusters of young stars in the galaxy, said Holley-Bockelmann. These so-called Central, Arches and Quintuplet clusters each contain hundreds of young, hot and huge stars. These typically burn out in “only” a few million years because of their extreme brightness, meaning there has been a relatively recent burst of star formation, according to the astronomers. A third piece of evidence is the supermassive black hole that dominates the center of the Milky Way itself, they added. A black hole is an object so heavy and compact that its gravity overwhelms any objects nearby; these eventually fall into the black hole. The one at the heart of the Milky Way is estimated to weigh the equivalent of about four million suns and to be about nine times as wide as the sun. Such an object produces intense gravitational tides. So astronomers were surprised to find a number of clumps of bright new stars closer than three lights years from the black hole’s maw. It wouldn’t be that surprising if the stars were being sucked into the black hole, but they show every sign of having formed in place, the astronomers said. For this to happen, the clouds of dust and gas that they formed from must have been thousands of times thicker than normally expected for such clouds. While there is an excess of young hot stars in the galactic core, there is also a surprising dearth of older stars, the researchers added. Theoretical models predict that there should be more old stars closer to the black hole, but there are very few within several light years of the sleeping giant. Holley-Bockelmann and colleagues came up with a theoretical model that they said fits the observations and makes some testable predictions. The story begins about 13 billion years ago, when one of the smaller satellite galaxies orbiting the Milky Way began drifting inward toward the core. Another recent study proposes this may have happened dozens of times in the Milky Way’s lifetime. As the satellite galaxy—a collection of stars and gas with a mid-sized black hole weighing about 10,000 suns—spiraled inward, most of its material was gradually stripped away, finally leaving the black hole and a handful of its satellite stars. About 10 million years ago, the satellite galaxy’s stripped-down core would have finally reached the galactic center. When two black holes merge, they first go through an elaborate dance. The smaller black hole would have circled the galactic black hole for several million years before it was ultimately gobbled up. As the smaller black hole circled closer and closer, it would have churned up the dust and gas in the vicinity and pushed enough material into the galactic black hole in the process to produce the Fermi bubbles, the researchers claim. And violent gravitational tides produced by the process could easily have compressed the molecular clouds in the core to super densities required to explain the many young stars. The vigorous churning would also have swept out the existing stars from the area surrounding the massive central black hole. In fact, the new model predicts that the black holes’ merger dance should have flung a large number of the missing old stars out into the galaxy at “hyper velocities,” explaining the absence of old stars in the area. “The gravitational pull of the satellite galaxy’s black hole could have carved nearly 1,000 stars out of the galactic center,” said Bogdanovic. “Those stars should still be racing through space, about 10,000 light years away from their original orbits.” If the theory works, it should be possible to identify these with large star surveys like the Sloan Digital Sky Survey, an ongoing project, she added.