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Bars may kill spiral galaxies

Nov. 8, 2010
Courtesy of the University of Portsmouth
and World Science staff

Helped by an ar­my of cit­i­zen vol­un­teers, sci­en­tists have con­clud­ed that ba­r-like struc­tures found in many spir­al ga­lax­ies—in­clud­ing our own—could be help­ing to de­stroy their grace­ful, twisty forms.

Most stars are part of ga­lax­ies, vast group­ings of stars con­tain­ing from a few hun­dred mil­lion to a quad­ril­lion of the fiery balls. Ga­lax­ies them­selves come in many shapes, in­clud­ing el­lip­ti­cal (wa­ter­mel­lon-shaped) and ir­reg­u­lar. Oth­ers are ma­jes­tic spir­als in which spir­al “arms” formed by stars and oth­er ma­te­ri­al wind out in a disk from a cen­tral bulge.

A barred spir­al gal­axy, NGC 3351. (Cred­it: NA­SA/JPL-Caltech/SSC )


About half of these spir­als al­so have a ba­r, a straight struc­ture cross­ing the cen­ter. Our own Milky Way gal­axy is a spir­al with a small ba­r.

Bars are be­lieved to strongly in­flu­ence the ev­o­lu­tion of ga­lax­ies as they pro­vide a way to move ma­te­ri­al in and out in the disk and pos­sibly help to spark star forma­t­ion in the cen­tral re­gions. They may even help feed the cen­tral mas­sive black hole that seems to be pre­s­ent in al­most all ga­lax­ies. 

But sci­en­tists don’t un­der­stand why some ga­lax­ies have ba­rs and oth­ers don’t.

In the new stu­dy, sci­en­tists drew on the work of vol­un­teers for Gal­axy Zoo 2, an on­line proj­ect in which mem­bers of the pub­lic are asked to care­fully clas­si­fy ga­lax­ies shown in pho­tos.

With these da­ta — which they called the larg­est ev­er sam­ple of ga­lax­ies with vis­u­al ba­r iden­ti­fica­t­ions — the re­search­ers, led by cos­mol­o­gist Ka­ren Mas­ters at the Uni­vers­ity of Ports­mouth in the U.K., de­ter­mined that red­dish spir­als are about twice as likely to host ba­rs as blu­ish spir­als. This mat­ters be­cause col­or is a clue to gal­axy age. “Blue” ga­lax­ies get their hue from the hot young stars they con­tain, im­ply­ing that they are form­ing many stars and are young. In “red” ga­lax­ies, this star forma­t­ion has stopped, leav­ing be­hind the cool­er, long-lived stars, which are red­der.

The as­tro­no­mers con­clude that ba­rs might help to kill spir­al ga­lax­ies, al­though how they do it re­mains a mys­tery. But the Milky Way has a ba­r too, so the find­ing may be tell­ing us some­thing about its fu­ture.

In a state­ment issued as the findings were revealed this week, Mas­ters was­n’t fo­cus­ing on any dis­may she might feel for the pos­sible aes­thet­ic de­mise of our home gal­axy. In­stead, she said it was “won­der­ful” to have “so many peo­ple in­volved in this re­search.”

“I feel a great weight of re­spon­si­bil­ity to make sure good sci­ence comes out of all the hard work they put in­to clas­si­fying ga­lax­ies,” she added. Da­ta hint­ing at the new re­sult has ex­isted for “some time,” she went on, but “with such a large num­ber of bar clas­sifica­t­ions we’re much more con­fi­dent about our re­sults. And all of this is thanks to the dedica­t­ion of the vol­un­teers who pro­vide the raw ‘clicks’.

“It’s not yet clear wheth­er the ba­rs are some side ef­fect of an ex­ter­nal pro­cess that turns spir­al ga­lax­ies red, or if they alone can cause this trans­forma­t­ion. We should get clos­er to an­swer­ing that ques­tion with more work on the Gal­axy Zoo da­taset.” 

A pa­per on the find­ings is to ap­pear in the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty and is al­so posted on the on­line da­tabase arX­iv.org.


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Helped by an army of citizen volunteers, scientists have concluded that bar-like structures found in many spiral galaxies—including our own—could be helping to destroy their graceful, twisty forms. Most stars are part of galaxies, vast groupings of stars containing from a few hundred million to a quadrillion of the fiery balls. Galaxies come in many shapes, including elliptical (American football-shaped) and irregular. Others are majestic spirals in which spiral “arms” formed by stars and other material wind out in a disk from a central bulge. About half of these spirals also have a bar, a straight structure crossing the center. Our own Milky Way galaxy is a spiral with a small bar. Bars are believed to strongly influence the evolution of galaxies as they provide a way to move material in and out in the disk and possibly help to spark star formation in the central regions. They may even help feed the central massive black hole that seems to be present in almost all galaxies. But scientists don’t understand why some galaxies have bars and others don’t. In the new study, scientists drew on the work of volunteers for Galaxy Zoo 2, an online project in which members of the public are asked to carefully classify galaxies displayed in photos, including information on the presence of a bar. With these data — which they called the largest ever sample of galaxies with visual bar identifications — the researchers, led by cosmologist Karen Masters at the University of Portsmouth in the U.K., determined that reddish spirals are about twice as likely to host bars as bluish spirals. This matters because color is a clue to galaxy age. “Blue” galaxies get their hue from the hot young stars they contain, implying that they are forming stars in large numbers. In “red” galaxies, this star formation has stopped, leaving behind the cooler, long-lived stars, which are redder. The astronomers conclude that bars might help to kill spiral galaxies, although how they do it remains a mystery. But the Milky Way has a bar too, so the finding may be telling us something about its future. But in a statement this week, Masters wasn’t focusing on any dismay she might feel for the possible aesthetic demise of our home galaxy. Instead, she noted that it was “wonderful” to have “so many people involved in this research.” “I feel a great weight of responsibility to make sure good science comes out of all the hard work they put into classifying galaxies,” she added. Data hinting at the new result have existed for “some time,” she went on, “with such a large number of bar classifications we’re much more confident about our results. And all of this is thanks to the dedication of the volunteers who provide the raw ‘clicks’. “It’s not yet clear whether the bars are some side effect of an external process that turns spiral galaxies red, or if they alone can cause this transformation. We should get closer to answering that question with more work on the Galaxy Zoo dataset.” A paper on the findings is to appear in the journal Monthly Notices of the Royal Astronomical Society and is also posted on the online database arXiv.org.