• Bars may kill spiral galaxies

  • 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.”

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