"Long before it's in the papers"
April 26, 2012

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Wall of stars found to extend through our galaxy

April 26, 2012
Courtesy of the Royal 
Astronomical Society, U.K.
and World Science staff

A wall of mini-ga­lax­ies and star clus­ters sur­rounds our gal­axy, as­tro­no­mers say—and poses a new prob­lem for the the­o­ry of “dark mat­ter,” an in­vis­i­ble sub­stance that per­me­ates ga­lax­ies.

The Milky Way, our gal­axy, con­tains some 300 bil­lion stars along with gas and dust clouds. These are ar­ranged roughly in a spir­al about 100,000 light-years wide, mean­ing it would take a light beam that much time to cross the dis­tance. 

An an­i­ma­tion il­lus­trates the lay­out of a "vast po­lar structure" de­tected by as­tro­no­mers around our gal­axy. A short blue hor­i­zon­tal line at the cen­ter rep­re­sents our gal­axy, seen edge­wise. Var­i­ous types of an­cil­lary struc­tures, in­clud­ing sat­el­lite ga­lax­ies, clus­ters of stars and streams of stars, are shown above and be­low. The an­i­ma­tion shows the whole thing spin­ning around so that its over­all ar­range­ment be­comes clear­er. Diff­er­ent cate­gories of ob­jects are added to the ani­mation as it pro­ceeds. (Cred­it: Mar­cel S. Paw­low­ski, U. of Bonn)
But our gal­axy is­n’t alone in its neigh­bor­hood: smaller sat­el­lite ga­lax­ies, and bunches of stars called glob­u­lar clus­ters, or­bit at var­i­ous dis­tances. 

Sci­en­tists have thought of these out­ly­ing stel­lar sub­urbs as some­what hap­haz­ardly ar­ranged, but the new study re­veals “a new pic­ture of our cos­mic neigh­bor­hood,” said Mar­cel Paw­low­ski, a doc­tor­al stu­dent from the Uni­vers­ity of Bonn in Ger­ma­ny who led the re­search. The find­ings are pub­lished in the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.

These smaller struc­tures are ar­ranged in a flat ar­ea, like a wall, ex­tend­ing above and be­low the spir­al and out­ward as far as a mil­lion light years, the study found. 

It’s not a tem­po­rary, hap­pen­stance ar­range­ment but a long-term struc­ture, he added, as il­lus­trat­ed by the pres­ence of “streams” of stars that al­so align with the wall. The stel­lar streams are rem­nants of move­ments and col­li­sions of var­i­ous larg­er ag­grega­t­ions of stars, he ex­plained, showing that all these acti­vities are re­strict­ed to the flat zone.

“The ob­jects are not only sit­u­at­ed with­in this plane right now, but... move with­in it,” so “the struc­ture is sta­ble,” Paw­low­ski ex­plained. “We were baf­fled by how well the dis­tri­bu­tions of the dif­fer­ent types of ob­jects agreed with each oth­er,” added Pa­vel Krou­pa, an as­tron­o­mer at the uni­vers­ity who col­la­bo­rat­ed on the stu­dy.

The wall is be­ing dubbed the “vast po­lar struc­ture” be­cause it ex­tends to­ward the poles of the spir­al gal­axy, or its spin ax­is.

The find­ings are al­so the sec­ond newly an­nounced re­sults within a mat­ter of days de­scribed as a ma­jor prob­lem for the widely held the­o­ry of dark mat­ter. Dark mat­ter is sup­posed to be an in­vis­i­ble sub­stance that makes up most of the mass, or weight, of the uni­verse. Main­stream as­tro­no­mers be­lieve dark mat­ter ex­ists be­cause there seems to be no oth­er way to ex­plain cer­tain gravita­t­ional forc­es de­tected op­er­at­ing across vast cos­mic dis­tances. But the new re­search sug­gests sci­en­tists may have to de­vise a new so­lu­tion to that prob­lem.

Dark mat­ter as­sump­tions have been built in­to the pre­vail­ing mod­els of how ga­lax­ies for­m—but these can­not easily ex­plain the new­found struc­ture sur­round­ing the Milky Way, Krou­pa said.

“In the stand­ard the­o­ries, the sat­el­lite ga­lax­ies would have formed as in­di­vid­ual ob­jects be­fore be­ing cap­tured by the Milky Way,” he ex­plained. “As they would have come from many di­rec­tions, it is next to im­pos­si­ble for them to end up dis­tribut­ed” as they are.

“The sat­el­lite ga­lax­ies and clus­ters must have formed to­geth­er in one ma­jor event, a col­li­sion of two ga­lax­ies,” ar­gued Jan Pflamm-Alt­en­burg, a postdoc­tor­al re­searcher and mem­ber of the re­search team. Such col­li­sions are fairly com­mon and lead to large chunks of ga­lax­ies be­ing torn out due to gravita­t­ional and tid­al forc­es act­ing on the stars, gas and dust they con­tain, form­ing tails that are the birth­places of new ob­jects like star clus­ters and dwarf ga­lax­ies.

“We think that the Milky Way col­lid­ed with anoth­er gal­axy in the dis­tant past. The oth­er gal­axy lost part of its ma­te­ri­al, ma­te­ri­al that then formed our Ga­laxy’s sat­el­lite ga­lax­ies and the young­er glob­u­lar clus­ters and the bulge at the ga­lac­tic cen­ter. The com­pan­ions we see to­day are the de­bris of this 11 bil­lion-year-old col­li­sion,” said Paw­low­ski.

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A wall of mini-galaxies and star clusters surrounds our galaxy, astronomers say—and poses a new problem for the theory of “dark matter,” an invisible substance that permeates galaxies. The Milky Way, our galaxy, contains some 300 billion stars along with gas and dust clouds arranged roughly in a spiral about 100,000 light-years wide, meaning it would take a light beam that much time to cross the distance. But our galaxy isn’t alone in its neighborhood: smaller satellite galaxies and clusters of stars, so-called globular clusters orbit at various distances. Scientists have thought these outlying stellar suburbs are somewhat haphazardly arranged, but the new study reveals “a new picture of our cosmic neighborhood,” said Marcel Pawlowski, a doctoral student from the University of Bonn in Germany who led the research. The findings are published in the journal Monthly Notices of the Royal Astronomical Society. These smaller structures are arranged in a flat area, like a wall, extending above and below the spiral and outward as far as one million light years, the study found. It’s not a temporary, happenstance arrangement but a long-term structure, he added, as illustrated by the presence of “streams” of stars that also align with the wall. The stellar streams are remnants of movements and collisions of various larger aggregations of stars, he explained, and show that these motions are restricted to the flat zone. “The objects are not only situated within this plane right now, but that they move within it,” so “the structure is stable,” Pawlowski explained. “We were baffled by how well the distributions of the different types of objects agreed with each other,” added Pavel Kroupa, an astronomer at the university who collaborated on the study. The wall is being dubbed the “vast polar structure” because it extends toward the poles of the spiral galaxy, or its spin axis. The findings are also the second newly announced results in a week described as a major problem for the widely held theory of dark matter. Dark matter is supposed to be an invisible substance that makes up most of the mass, or weight, of the universe. Mainstream astronomers believe dark matter exists because there seems to be no other way to explain certain gravitational forces detected operating across vast distances in the universe. But the new research suggests scientists may have to devise a new solution to that problem. Dark matter assumptions have been built into the prevailing models of how galaxies form—but these cannot easily explain the newfound structure surrounding the Milky Way, Kroupa said. “In the standard theories, the satellite galaxies would have formed as individual objects before being captured by the Milky Way,” he explained. “As they would have come from many directions, it is next to impossible for them to end up distributed” as they are. “The satellite galaxies and clusters must have formed together in one major event, a collision of two galaxies,” argued Jan Pflamm-Altenburg, a postdoctoral researcher and member of the research team. Such collisions are fairly common and lead to large chunks of galaxies being torn out due to gravitational and tidal forces acting on the stars, gas and dust they contain, forming tails that are the birthplaces of new objects like star clusters and dwarf galaxies. “We think that the Milky Way collided with another galaxy in the distant past. The other galaxy lost part of its material, material that then formed our Galaxy’s satellite galaxies and the younger globular clusters and the bulge at the galactic centre. The companions we see today are the debris of this 11 billion year old collision,” said Pawlowski.