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"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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