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Explosion shutting down a galactic party: physicists

Feb. 23, 2011
Courtesy of the Gemini Observatory
and World Science staff

An im­mense black hole in a gal­axy far, far away seems to be caus­ing an ex­plo­sion that will change that gal­axy for­ev­er, sci­en­tists say.

Blasts of its type, nev­er re­ported in de­tail be­fore, will snuff out a ga­lac­tic par­ty, they pre­dict: like a sud­denly en­raged fa­ther who at a hol­i­day feast yanks the ta­ble­cloth high in­to the air, the black hole is in a sense toss­ing its own meals out of the gal­axy. And eve­ry­one else’s, too, be­cause some of this same ma­te­ri­al is ex­pected to have been nour­ish­ing a flur­ry of new star forma­t­ion else­where in the gal­axy, which will now stop.

Artist’s con­cep­tu­al­ of the en­vi­ron­ment around the su­pe­r­mas­sive black hole at the cen­ter of Mrk 231. The broad out­flow seen in the Gem­i­ni da­ta is shown as the fan-shaped wedge at the top of the ac­cre­tion disk around the black hole. This side-view is not what is seen from the Earth where we see it ‘look­ing down the throat’ of the out­flow. A si­m­i­lar out­flow is prob­a­bly pre­s­ent un­der the disk as well and is hinted at in this il­lus­tra­tion. The to­tal amount of ma­te­ri­al en­trained in the broad flow is at least 400 times the mass of the Sun per year. Note that a more lo­cal­ized, nar­rower je­t is shown, this je­t was known pri­or to the Gem­i­ni dis­cov­ery of the broader out­flow fea­tured he­re. (Cour­tesy of Gem­i­ni Ob­ser­va­to­ry, art­work by Lyn­ette Cook )


“This is really a last gasp of this gal­axy. The black hole is belch­ing its next meals in­to obliv­ion!” said Syl­vain Veilleux of the Uni­vers­ity of Mar­y­land, one of the au­thors of the re­search. The re­port is to ap­pear in the March 10 is­sue of The As­t­ro­phys­i­cal Jour­nal Let­ters.

A black hole is an ob­ject with an enor­mous weight packed in­to such a small space that it de­vel­ops an in­vin­ci­ble gravita­t­ional force in its im­me­di­ate sur­round­ings. An­y­thing that gets too close, even beams of light, gets sucked in.

Giant, or “su­pe­r­mas­sive,” black holes sit at the cen­ters of most ga­lax­ies like an­chors, help­ing to keep their stars tied to the sys­tem. All the while, phys­i­cists say, these black holes eat what­ev­er strays near­by—some­times just nib­bling, oth­er times feast­ing, in a vi­o­lent pro­cess that al­so re­leases heat and light.

The situa­t­ion can change when the gal­axy finds it­self merg­ing with anoth­er one, ac­cord­ing to re­search­ers. Then waves of new ma­te­ri­al, in­clud­ing gas and dust, wash in­to the gal­axy. This spurs the forma­t­ion of new stars and al­so pro­vides fresh fod­der for the cen­tral black hole, dras­tic­ally boost­ing its in­take. 

But these events prove their own un­do­ing, phys­i­cists say. The vo­ra­cious con­sump­tion pro­cess starts to cre­ate so much heat that it gen­er­ates a fierce light. This light in turn pushes out­ward against the gas and dust sur­round­ing the black hole—its food. Turn up the heat enough, and the light blows the whole din­ner away. Not only is the black hole’s meal dis­rupted, but star forma­t­ion through­out the gal­axy, al­so nour­ished by this ma­te­ri­al, grinds to a near-halt.

Pre­vi­ous stud­ies have ob­served a com­mon var­i­ant of this pro­cess: rath­er than the whole ga­lac­tic sup­pe­r be­ing dis­rupted, beams of light called col­li­mat­ed jets merely punch small holes through the ma­te­ri­al around the black hole. But a full-on blow­out, sci­en­tists say, is new to our ob­serva­t­ions. 

In the gal­axy that they de­scribe, they add, both pro­cesses are in play, the co­coon be­ing pushed out­ward at the same time as holes are punched straight through it. While this may be hard to pic­ture, it is eas­i­er when we con­sid­er that such large-scale ex­plo­sions ap­pear to hap­pen very slowly from our pe­r­spec­tive, ow­ing to the vast­ness of the spaces in­volved. The ex­plod­ing ma­te­ri­al moves in­con­ceiv­ably fast, but due to the huge amounts of space that it has to fill, it looks slow from Earth.

Though this chart is less im­pres­sive than the art­work, it is part of the ac­tu­al tel­e­scope da­ta that re­flects the same type of event, sci­en­tists say. It is a cross-sec­tion of a three-dim­ension­al "da­ta cube" show­ing in­for­ma­tion from throug­hout the gal­axy, in par­tic­u­lar the large-scale, fast out­flow of neu­tral so­di­um gas, detect­able be­cause it ab­sorbs yel­low light. The view­er is look­ing down on­to the ma­te­ri­al that moves to­ward us rel­a­tive to the gal­axy. The large black cir­cle marks the lo­ca­tion of the black hole, and red lines show the lo­ca­tion of a radio-wave je­t. In ad­di­tion to the qua­sar out­flow, the je­t pushes the ma­te­ri­al at the top right, re­sult­ing in even great­er speeds. Part of the star­burst is lo­cat­ed at the po­si­tion of the box at the low­er left, and it is like­ly re­spon­si­ble for the gas mo­tion in this re­gion, re­search­ers say. (Cour­te­sy of Gem­i­ni Ob­serv­a­to­ry)


Veilleux and Da­vid Rupke of Rhodes Col­lege in Ten­nes­see used a tel­e­scope at the Gem­i­ni Ob­serv­a­to­ry in Mauna Kea, Ha­waii, to study what they said were events in a gal­axy known as Markar­ian 231. “This ob­ject is ar­guably the clos­est and best ex­am­ple that we know of a big gal­axy in the fi­nal stages of a vi­o­lent merg­er and in the pro­cess of shed­ding its co­coon and re­veal­ing a very en­er­get­ic cen­tral qua­sar,” anoth­er term for a highly ac­tive black hole, Veilleux said.

As ex­treme as Markar­ian 231’s eat­ing habits seem, Veilleux said they’re probably not un­ique. “When we look deep in­to space and back in time, qua­sars like this one are seen in large num­bers, and all of them may have gone through shed­ding events like the one we are wit­ness­ing.” 

Markar­ian 231 is well known for its col­li­mat­ed jets, but the new study al­so re­vealed a broad “out­flow” ex­tend­ing in all di­rec­tions around the gal­ax­y’s co­re, the two sci­en­tists said. The gas is es­ti­mat­ed to be mov­ing at over 1,000 kilo­me­ters (600 miles) per sec­ond. At this speed, the gas could go from New York to Los An­ge­les in about four sec­onds. The en­er­gy to pow­er the mas­sive out­flow can only come from the black hole, be­cause stars don’t have enough en­er­gy to drive such a pro­cess, they added.

“The fire­works of new star forma­t­ion and black hole feed­ing are com­ing to an end, most likely as a re­sult of this out­flow,” Rupke said. 

The en­vi­ron­ment around such a black hole is com­monly known as an ac­tive ga­lac­tic nu­cle­us. Merg­ing ga­lax­ies help to feed the cen­tral black hole and al­so shroud it in gas. But even­tu­al­ly, bereft of fu­el, the ac­tive ga­lac­tic nu­cle­us goes “ex­tinct,” scient­ists say. With­out gas to form new stars, the host gal­axy al­so starves, turn­ing in­to a col­lec­tion of ag­ing stars. Ul­ti­mate­ly, these old stars will make the gal­axy ap­pear red­der, giv­ing these ga­lax­ies the mon­i­ker “red and dead.” 

Nu­mer­i­cal as­t­ro­phys­i­cist Phil­ip Hop­kins of the Uni­vers­ity of Cal­i­for­nia at Berke­ley said pro­cesses un­ique to fast-grow­ing black holes probably help pro­pel the winds ob­served by Gem­i­ni. These ex­tend an es­ti­mat­ed 8,000 light years out­ward, a light year be­ing the dis­tance that light trav­els in a year. “At its peak, the qua­sar shines with such in­tens­ity that the light it­self is ‘trapped’ by a co­coon of gas and dust,” Hop­kins ex­plained. This light pushes “on ma­te­ri­al with a force that can easily overcome the gravita­t­ional pull of the black hole.”

Hop­kins adds that the bath of X-rays and gam­ma rays known to be gener­ated by qua­sars could al­so heat up the gas in the gal­ax­y’s cen­ter un­til it reaches a tempe­rature where it “boils over” and causes a bomb-like ex­plo­sion. “But un­til now, we haven’t been able to catch a sys­tem ‘in the ac­t.’” Part of the prob­lem, ac­cord­ing to Hop­kins, has been that the most vis­i­ble out­flows are those ‘col­li­mat­ed jets’ al­ready known in Markar­ian 231. These jets are trapped in­to nar­row beams, probably by mag­net­ic fields, where­as ma­te­ri­al falls in­to the black hole from all di­rec­tions.

Markar­ian 231 lies an es­ti­mat­ed 600 mil­lion light-years away in the di­rec­tion of the con­stella­t­ion of Ur­sa Ma­jor. Some es­ti­mates in­di­cate that the gal­axy has a weight in stars of about three times that of our Milky Way gal­axy, while its cen­tral black hole is es­ti­mat­ed to weigh the equiv­a­lent of about 10 mil­lion of our suns, or al­so about three times the supe­rmas­sive black holes in our own gal­axy.


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An immense black hole in a galaxy far, far away seems to be causing a colossal explosion that will change the galaxy forever, scientists say. Blasts like this, never reported before, will snuff out a galactic party, they predict: like a suddenly enraged father at a holiday feast who yanks the tablecloth high into the air, the black hole is in a sense tossing its own meals out of the galaxy. And everyone else’s, too, because some of this same material is expected to have been nourishing a flurry of new star formation elsewhere in the galaxy, which will now stop. “This is really a last gasp of this galaxy. The black hole is belching its next meals into oblivion!” said Sylvain Veilleux of the University of Maryland, one of the authors of the new research, to appear in the March 10 issue of The Astrophysical Journal Letters. A black hole is an object with an enormous weight packed into such a small space that it develops an invincible gravitational force in its immediate surroundings. Anything that gets too close, even beams of light, gets sucked in. Unusually large, or “supermassive,” black holes sit at the centers of most galaxies like anchors, helping to keep their stars tied to the system. All the while, physicists say, these black holes eat whatever strays nearby them—sometimes just nibbling, other times feasting, in a violent process that also releases heat and light. The situation can change when the galaxy finds itself merging with another one, according to researchers. Then waves of new material, including gas and dust, wash into the galaxy. This spurs the formation of new new stars and also provides new fodder for the central black hole, drastically boosting its intake. But these events prove their own undoing, physicists say. The voracious consumption process starts to create so much heat that it generates a fierce light. This light in turn pushes outward against the gas and dust surrounding the black hole—its food. Turn up the heat enough, and the light blows the whole dinner away. Not only is the black hole’s meal disrupted, but the rampant star formation throughout the galaxy, also nourished by this material, comes to a near-halt. Previous studies have observed a common variant of this process: rather than the whole galactic supper being disrupted, beams of light called collimated jets merely punch small holes through the surrounding cocoon of material. But a full-on blowout, scientists say, is new to our observations. In the galaxy that they describe, they say, both processes are occuring at once, the cocoon being pushed outward at the same time as holes are punched straight through it. While this may be hard to picture, it is easier when we consider that such large-scale explosions appear to happen very slowly from our perspective, owing to the vastness of the spaces involved. The exploding material moves blindingly fast, but due to the huge amounts of space that it has to fill, the process looks slow from Earth. Veilleux and David Rupke of Rhodes College in Tennessee used a telescope at the Gemini Observatory in Mauna Kea, Hawaii, to study what they said were events in a galaxy known as Markarian 231. “This object is arguably the closest and best example that we know of a big galaxy in the final stages of a violent merger and in the process of shedding its cocoon and revealing a very energetic central quasar,” another term for a highly active black hole, Veilleux said. As extreme as Markarian 231’s eating habits seem, Veilleux said they’re probably not unique. “When we look deep into space and back in time, quasars like this one are seen in large numbers, and all of them may have gone through shedding events like the one we are witnessing.” Markarian 231 is well known for its collimated jets, but the new study also revealed a broad “outflow” extending in all directions around the galaxy’s core, the two scientists said. The gas is estimated to be moving at over 1,000 kilometers (600 miles) per second. At this speed, the gas could go from New York to Los Angeles in about four seconds. The energy to power the massive outflow can only come from the black hole, because stars don’t have enough energy to drive such a process, they added. “The fireworks of new star formation and black hole feeding are coming to an end, most likely as a result of this outflow,” Rupke said. The environment around such a black hole is commonly known as an active galactic nucleus. Merging galaxies help to feed the central black hole and also shroud it in gas. Markarian 231 is in transition, now clearing its surroundings, according to Rupke and Veilleux. Eventually, running out of fuel, the active galactic nucleus will go “extinct.” Without gas to form new stars, the host galaxy also starves, turning into a collection of old aging stars with few young stars to regenerate the stellar population. Ultimately, these old stars will make the galaxy appear redder, giving these galaxies the moniker “red and dead.” Numerical astrophysicist Philip Hopkins of the University of California at Berkeley said processes unique to fast-growing black holes probably help propel the winds observed by Gemini. These extend an estimated 8,000 light years outward, a light year being the distance that light travels in a year. “At its peak, the quasar shines with such intensity that the light itself is ‘trapped’ by a cocoon of gas and dust,” Hopkins explained. This light pushes “on material with a force that can easily overcome the gravitational pull of the black hole.” Hopkins adds that the bath of X-rays and gamma rays known to be generated by quasars could also heat up the gas in the galaxy’s center until it reaches a temperature where it “boils over” and causes a bomb-like explosion. “But until now, we haven’t been able to catch a system ‘in the act.’” Part of the problem, according to Hopkins, has been that the most visible outflows are those ‘collimated jets’ already known in Markarian 231. These jets are trapped into narrow beams, probably by magnetic fields, whereas material falls into the black hole from all directions. Markarian 231 lies an estimated 600 million light-years away in the direction of the constellation of Ursa Major. Some estimates indicate that the galaxy has a weight in stars of about three times that of our Milky Way galaxy, while its central black hole is estimated to weigh the equivalent of about 10 million of our suns, or also about three times the supermassive black holes in our own galaxy.