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“Strong evidence” of jet at Milky Way center

Nov. 21, 2013
Courtesy of Chandra X-Ray Observatory
and World Science staff

Af­ter a long search, as­tro­no­mers have found “strong ev­i­dence” that the huge black hole at the cen­ter of our Milky Way gal­axy is shoot­ing out a jet of high-en­er­gy par­t­i­cles. 

Pre­vi­ous stud­ies sug­gested there was a jet, but the re­ports of­ten con­tra­dicted each oth­er, weren’t con­sid­ered de­fin­i­tive and were un­clear on its di­rec­tion.

This com­pos­ite im­age fea­tures both X-rays from Chan­dra (pur­ple) and ra­di­o da­ta from the VLA (blue). As the faint­ly vis­i­ble je­t fires away from Sgr A*, it trav­els through un­til it hits gas sev­er­al light years away. (The re­gion around the Milky Way's black hole has many clumps of gas and dust.) Once the je­t hits, it trig­gers the for­ma­tion of a "shock front." This in­ter­ac­tion al­so ac­cel­er­ates elec­trons, gen­er­at­ing X-rays as the elec­trons stream down the path of the je­t, past the shock front. (Cred­it: X-ray: NA­SA/CX­C/U­CLA/Z.Li et al; Ra­di­o: NRAO/VLA)


“For dec­ades as­tro­no­mers have looked for a jet as­so­ci­at­ed with the Milky Way’s black hole. Our new ob­serva­t­ions make the strongest case yet” for it, said Zhi­yuan Li of Nan­jing Uni­vers­ity in Chi­na, lead au­thor of a study to ap­pear in an up­com­ing edi­tion of The As­t­ro­phys­i­cal Jour­nal and on­line now.

The “su­per­mas­sive” or gi­ant black hole at the cen­ter of the gal­axy is called Sag­it­ta­ri­us A*. A black hole is an ob­ject so com­pact that its gra­vity sucks in an­ything that passes near­by, even light. But some of the in­falling ma­te­ri­al may al­so be redi­rected out­ward, form­ing jets. 

Most ga­lax­ies are be­lieved to host su­per­mas­sive black holes at their cen­ters, and these play im­por­tant roles in the gal­ax­y’s over­all ev­o­lu­tion.

Jets of high-en­er­gy par­t­i­cles are al­so found through­out the uni­verse, in many sizes. They come from young stars and from black holes a thou­sand times larg­er than the Milky Way’s black hole. They car­ry en­er­gy away from the cen­tral ob­ject and, on a ga­lac­tic scale, in­flu­ence the forma­t­ion rate of new stars.

The new find­ings come from NASA’s Chan­dra X-ray Ob­serv­a­to­ry and the Na­t­ional Sci­ence Founda­t­ion’s Very Large Ar­ray ra­di­o tel­e­scope.

“We were very ea­ger to find a je­t from Sag­it­ta­ri­us A* be­cause it tells us the di­rec­tion of the black hole’s spin ax­is. This gives us im­por­tant clues about the growth his­to­ry of the black hole,” said Mark Mor­ris of the Uni­vers­ity of Cal­i­for­nia at Los An­ge­les, a co-au­thor of the stu­dy.

The study in­di­cates the black hole is spin­ning in the same plane as the rest of the gal­axy. That tells as­tro­no­mers ma­te­ri­al has mi­grat­ed steadily in­to the black hole over the past 10 bil­lion years. If the Milky Way had col­lid­ed with large ga­lax­ies in the re­cent past and their cen­tral black holes had merged, the jet could point in any di­rec­tion.

The jet is weak, and seems to be run­ning in­to gas near the black hole—there pro­duc­ing X-rays and ra­di­o waves, sci­en­tists said. The two key pieces of ev­i­dence for the jet, they added, are a straight line of X-ray emit­ting gas that points to­ward the black hole and a “shock front” where the jet seems to be strik­ing the gas, seen in ra­di­o data—si­m­i­lar to a son­ic boom. Ad­di­tion­al­ly, the en­er­gy dis­tri­bu­tion of X-rays from the black hole re­sem­bles that of je­ts com­ing from su­per­mas­sive black holes in oth­er ga­lax­ies.

Since Sag­it­ta­ri­us A* is known to be con­sum­ing very lit­tle ma­te­ri­al, it’s not sur­pris­ing that the je­t ap­pears weak, as­tro­no­mers said. A jet in the op­po­site di­rec­tion, though of­ten seen, is not in this case, pos­sibly be­cause of gas or dust block­ing the line of sight from Earth or a lack of ma­te­ri­al to fu­el the jet.

The re­gion around the black hole is faint, which means the black hole has been qui­et in the past few hun­dred years, they added. A sep­a­rate Chan­dra study an­nounced last month shows that it was at least a mil­lion times brighter be­fore then.

“We know this gi­ant black hole has been much more ac­tive at con­sum­ing ma­te­ri­al in the past. When it stirs again, the je­t may bright­en dra­mat­ic­ally,” said co-au­thor Fred­er­ick K. Baganoff of the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy in Cam­bridge, Mass.

As­tro­no­mers have sug­gested the gi­ant bub­bles of high-en­er­gy par­t­i­cles ex­tend­ing out from the Milky Way and de­tected by NASA’s Fer­mi Gam­ma Ray Tel­e­scope in 2008 are caused by je­ts from the black hole that are aligned with the spin ax­is of the gal­axy. The lat­est re­sults from Chan­dra are be­lieved to sup­port this.

Sag­it­ta­ri­us A* is about four mil­lion times more mas­sive than our Sun and lies about 26,000 light-years from Earth. A light-year is the dis­tance light trav­els in a year.


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After a long search, astronomers found “strong evidence” that the huge black hole at the center of our Milky Way galaxy is shooting out a jet of high-energy particles. Previous studies suggested there was a jet, but the reports often contradicted each other, weren’t considered definitive and were unclear on the direction of any jet. “For decades astronomers have looked for a jet associated with the Milky Way’s black hole. Our new observations make the strongest case yet” for it, said Zhiyuan Li of Nanjing University in China, lead author of a study to appear in an upcoming edition of The Astrophysical Journal and online now. The “supermassive” or giant black hole at the center of the galaxy is called Sagittarius A*. A black hole is an object so compact that its gravity sucks in anything that passes nearby, even light. But some of the infalling material may also be redirected outward, forming jets. Most galaxies are believed to host supermassive black holes at their centers, and these play important roles in the galaxy’s overall evolution. Jets of high-energy particles are also found throughout the universe, in many sizes. They come from young stars and black holes a thousand times larger than the Milky Way’s black hole. They carry energy away from the central object and, on a galactic scale, influence the formation rate of new stars. The new findings come from NASA’s Chandra X-ray Observatory and the National Science Foundation’s Very Large Array radio telescope. “We were very eager to find a jet from Sagittarius A* because it tells us the direction of the black hole’s spin axis. This gives us important clues about the growth history of the black hole,” said Mark Morris of the University of California at Los Angeles, a co-author of the study. The study indicates the black hole is spinning in the same plane as the rest of the galaxy. That tells astronomers material has migrated steadily into the black hole over the past 10 billion years. If the Milky Way had collided with large galaxies in the recent past and their central black holes had merged, the jet could point in any direction. The jet seems to be running into gas near the black hole, producing X-rays detected by Chandra and radio emission, scientists said. The two key pieces of evidence for the jet, they added, are a straight line of X-ray emitting gas that points toward the black hole and a “shock front” where the jet seems to be striking the gas, seen in radio data—similar to a sonic boom. Additionally, the energy distribution of X-rays from the black hole resembles that of jets coming from supermassive black holes in other galaxies. Since Sagittarius A* is known to be consuming very little material, it’s not surprising that the jet appears weak, astronomers said. A jet in the opposite direction, though often seen, is not in this case, possibly because of gas or dust blocking the line of sight from Earth or a lack of material to fuel the jet. The region around the black hole is faint, which means the black hole has been quiet in the past few hundred years, they added. However, a separate Chandra study announced last month shows that it was at least a million times brighter before then. “We know this giant black hole has been much more active at consuming material in the past. When it stirs again, the jet may brighten dramatically,” said co-author Frederick K. Baganoff of the Massachusetts Institute of Technology in Cambridge, Mass. Astronomers have suggested the giant bubbles of high-energy particles extending out from the Milky Way and detected by NASA’s Fermi Gamma Ray Telescope in 2008 are caused by jets from the black hole that are aligned with the spin axis of the galaxy. The latest results from Chandra are believed to support this. Sagittarius A* is about four million times more massive than our Sun and lies about 26,000 light-years from Earth. A light-year is the distance light travels in a year.