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Our “home” black hole’s last big blast dated to 2 million years ago

Sept. 22, 2013
Courtesy of the Australian Astronomical Observatory
and World Science staff

A dor­mant “vol­cano”—a gi­ant black hole—lies at the heart of our gal­axy. Fresh ev­i­dence sug­gests it last erupted two mil­lion years ago, as­tro­no­mers say.

The ev­i­dence, they say, comes from a lacy thread of gas, mostly hy­dro­gen, called the Mag­el­lanic Stream, trail­ing our gal­ax­y’s two small com­pan­ion ga­lax­ies, the Large and Small Mag­el­lanic Clouds.

An artist’s con­cep­tion of a black hole gen­er­at­ing a jet. (Cred­it: NA­SA/­Dana Berry/Sky­Works Dig­it­al)


“For 20 years we’ve seen this odd glow from the Mag­el­lanic Stream,” said Joss Bland-Hawthorn of Uni­vers­ity of Syd­ney, Aus­tral­ia and the Aus­tral­ian As­tro­nom­i­cal Ob­serv­a­to­ry, who led a team stu­dy­ing the prob­lem. “We did­n’t un­der­stand the cause. Then sud­denly we real­ized it must be the mark, the fos­sil rec­ord, of a huge out­burst of en­er­gy from the cen­ter of our gal­axy.”

The find­ings are de­scribed in a pa­per ac­cept­ed for pub­lica­t­ion in The As­t­ro­phys­i­cal Jour­nal. Bland-Hawthorn will speak about the work at the Gal­axy Zoo meet­ing in Syd­ney, Aus­tral­ia, on Sept. 24.

A black hole is an ob­ject so com­pact that its gra­vity be­comes over­whelm­ing and draws in an­y­thing that strays too close, in­clud­ing light. An erup­tion of en­er­gy oc­curs when the black hole swal­lows a large amount of ma­te­ri­al.

Most ga­lax­ies are be­lieved to har­bor a gi­ant black hole at their cen­ters, called a su­per­mas­sive black hole. The one at the heart of our gal­axy, the Milky Way, has been known for dec­ades. A swarm of stars or­bit it; their paths let us “weigh” the black hole, or tech­nic­ally, meas­ure its mass. That is es­ti­mat­ed to be the equiv­a­lent of four mil­lion Suns.

The re­gion around the black hole, called Sag­it­ta­ri­us A* [“A-star”], pours out radia­t­ion in the form of ra­di­o waves, in­fra­red, X-rays and gam­ma rays. Flick­ers of this radia­t­ion rise up when small clouds of gas fall on­to the hot disk of mat­ter that swirls around the black hole.

But ev­i­dence has been build­ing of a real cat­a­clysm in the past, as­tro­no­mers say. In­fra­red and X-ray satel­lites have seen a pow­er­ful out­flow of ma­te­ri­al from this cen­tral re­gion. Ma­te­ri­al known as an­ti­mat­ter boil­ing out has left its sig­na­ture. And there are the “Fermi bub­bles”—two huge hot bub­bles of gas bil­low­ing out from the ga­lac­tic cen­ter, seen in gam­ma-rays and ra­di­o waves.

“All this points to a huge ex­plo­sion at the cen­ter of our gal­axy,” said team mem­ber Phil­ip Mal­oney of the Uni­vers­ity of Col­o­rad­o at Boul­der. Such a blast is called a Seyfert flare.

Sci­en­tists stu­dy­ing the ga­lac­tic cen­ter came to­geth­er at a work­shop at Stan­ford Uni­vers­ity in Cal­i­for­nia ear­li­er this year. While there, Pro­fes­sor Bland-Hawthorn said he real­ized the Stream could be hold­ing the mem­o­ry of the ga­lac­tic cen­ter’s past. Struck by the fiery breath of Sag­it­ta­ri­us A*, he argues, the Stream is emit­ting light, much as par­t­i­cles from the Sun hit our at­mos­phere and trig­ger the col­ored glows in the sky called the North­ern and South­ern Lights.

In the Stream, ul­tra­vi­o­let light splits hy­dro­gen atoms in­to their parts, pro­tons and elec­trons. When those com­po­nents re­com­bine, the elec­trons give off “H-alpha” emis­sion—a spe­cif­ic wave­length, or “col­or,” of light. The bright­est glow in the Stream comes from the re­gion near­est the ga­lac­tic cen­ter.

Ge­om­e­try, the amount of en­er­gy from the orig­i­nal flare from Sag­it­ta­ri­us A*, the time the flare would take to trav­el to the Mag­el­lanic Stream, the rate at which the Stream would have cooled over time—“it all fits to­geth­er, it all adds up,” said team mem­ber Greg Mad­sen of the Uni­vers­ity of Cam­bridge in the UK.

The gal­ax­y’s stars don’t pro­duce enough ul­tra­vi­o­let to ac­count for the glow. Nor could they have in the past, said Bland-Hawthorn. “The ga­lac­tic cen­ter nev­er formed stars at a high enough rate.”

Will such an ex­plo­sion hap­pen again? “There are lots of stars and gas clouds that could fall on­to the hot disk around the black hole,” said Bland-Hawthorn. “There’s a gas cloud called G2 that we think will fall in next year. It’s small, but we’re look­ing for­ward to the fire­work­s!”


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A dormant “volcano”—a giant black hole—lies at the heart of our galaxy. Fresh evidence suggests it last erupted two million years ago, astronomers say. The evidence, they say, comes from a lacy thread of gas, mostly hydrogen, called the Magellanic Stream, trailing our galaxy’s two small companion galaxies, the Large and Small Magellanic Clouds. “For 20 years we’ve seen this odd glow from the Magellanic Stream,” said Joss Bland-Hawthorn of University of Sydney, Australia and the Australian Astronomical Observatory, who led a team studying the problem. “We didn’t understand the cause. Then suddenly we realized it must be the mark, the fossil record, of a huge outburst of energy from the center of our galaxy.” The findings are described in a paper accepted for publication in The Astrophysical Journal. Bland-Hawthorn will speak about the work at the Galaxy Zoo meeting in Sydney, Australia, on Sept. 24. A black hole is an object so compact that its gravity becomes overwhelming and draws in anything that strays too close, including light. An eruption of energy occurs when the black hole swallows a large amount of material. Most galaxies are believed to harbor a giant black hole at their centers, called a supermassive black hole. The one at the heart of our galaxy, the Milky Way, has been known for decades. A swarm of stars orbit it; their paths let us “weigh” the black hole, or technically, measure its mass. That is estimated to be the equivalent of four million Suns. The region around the black hole, called Sagittarius A* [“A-star”], pours out radiation in the form of radio waves, infrared, X-rays and gamma rays. Flickers of this radiation rise up when small clouds of gas fall onto the hot disk of matter that swirls around the black hole. But evidence has been building of a real cataclysm in the past, astronomers say. Infrared and X-ray satellites have seen a powerful outflow of material from this central region. Material known as antimatter boiling out has left its signature. And there are the “Fermi bubbles”—two huge hot bubbles of gas billowing out from the galactic center, seen in gamma-rays and radio waves. “All this points to a huge explosion at the center of our galaxy,” said team member Philip Maloney of the University of Colorado at Boulder. “What astronomers call a Seyfert flare.” Scientists studying the galactic center came together at a workshop at Stanford University in California earlier this year. While there, Professor Bland-Hawthorn said he realized the Stream could be holding the memory of the galactic center’s past. Struck by the fiery breath of Sagittarius A*, the Stream is emitting light, much as particles from the Sun hit our atmosphere and trigger the colored glows of the aurorae—the Northern and Southern Lights. In the Stream, ultraviolet light splits hydrogen atoms into their parts, protons and electrons. When those components recombine, the electrons give off “H-alpha” emission—a specific wavelength, or “color,” of light. The brightest glow in the Stream comes from the region nearest the galactic center. Geometry, the amount of energy from the original flare from Sagittarius A*, the time the flare would take to travel to the Magellanic Stream, the rate at which the Stream would have cooled over time—“it all fits together, it all adds up,” said team member Greg Madsen of the University of Cambridge in the UK. The galaxy’s stars don’t produce enough ultraviolet to account for the glow. Nor could they have in the past, said Bland-Hawthorn. “The galactic center never formed stars at a high enough rate.” Will such an explosion happen again? “There are lots of stars and gas clouds that could fall onto the hot disk around the black hole,” said Bland-Hawthorn. “There’s a gas cloud called G2 that we think will fall in next year. It’s small, but we’re looking forward to the fireworks!”