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Mysterious dance of dwarfs may force a cosmic rethink

July 21, 2014
Courtesy of the University of Sydney
and World Science staff

A find­ing that many small ga­lax­ies don’t “swarm” around larg­er ones like bees but rath­er circle them in disc-shaped or­bits is cre­at­ing a new co­nun­drum for sci­en­tists.

Last year “we an­nounced our startling dis­cov­ery that half of the dwarf ga­lax­ies sur­round­ing the An­drom­e­da Gal­axy are or­bit­ing it in an im­mense plane,” said phys­i­cist Ge­raint Lew­is from the Uni­vers­ity of Syd­ney in Aus­tral­ia. “This plane is more than a mil­lion light years in di­am­e­ter, but is very thin, with a width of only 300,000 light years.”

Now, as­tro­no­mers are ex­tend­ing the find­ing to oth­er ga­lax­ies.

The uni­verse con­tains bil­lions of ga­lax­ies. Some, such as the Milky Way, are im­mense, con­tain­ing hun­dreds of bil­lions of stars. Most ga­lax­ies, how­ev­er, are dwarfs, much smaller and with only a few bil­lion stars. For dec­ades as­tro­no­mers have used com­put­er mod­els to pre­dict how these dwarf ga­lax­ies should or­bit large ga­lax­ies. They had al­ways found that they should be scat­tered ran­dom­ly.

“Our An­drom­e­da dis­cov­ery did not agree with ex­pecta­t­ions, and we felt com­pelled to ex­plore if it was true of oth­er ga­lax­ies through­out the uni­verse,” said Lew­is. Us­ing the Sloan Dig­it­al Sky Sur­vey, a re­source of col­or im­ages and 3-D maps cov­er­ing more than a third of the sky, the re­search­ers dis­sect­ed the prop­er­ties of thou­sands of near­by ga­lax­ies.

“We were sur­prised to find that a large pro­por­tion of pairs of sat­el­lite ga­lax­ies have op­po­sitely di­rect­ed ve­lo­ci­ties if they are sit­u­at­ed on op­po­site sides of their gi­ant gal­axy hosts,” said Neil Ibata of the Ly­cée In­terna­t­ional in Stras­bourg, France, lead au­thor of a new study on the find­ings, pub­lished July 20 in the jour­nal Na­ture.

“Ever­ywhere we looked we saw this strangely co­her­ent co­or­di­nated mo­tion of dwarf ga­lax­ies. From this we can ex­trap­o­late that these cir­cu­lar planes of danc­ing dwarfs are uni­ver­sal, seen in about 50 per­cent of ga­lax­ies,” said Lew­is. “This is a big prob­lem that con­tra­dicts our stand­ard cos­mo­lo­g­i­cal mod­els. It chal­lenges our un­der­stand­ing of how the uni­verse works in­clud­ing the na­ture of dark mat­ter,” an un­seen ma­te­ri­al that is de­tected through its gra­vity.

The re­search­ers be­lieve the an­swer may be hid­den in some cur­rently un­known phys­i­cal pro­cess that gov­erns how gas flows in the uni­verse. Some ex­perts have made more rad­i­cal sug­ges­tions, in­clud­ing bend­ing and twist­ing the laws of gra­vity and mo­tion. “Throw­ing out seem­ingly es­tab­lished laws of phys­ics is un­palat­able,” said Pro­fes­sor Lew­is, “but if our ob­serva­t­ions of na­ture are point­ing us in this di­rec­tion, we have to keep an open mind. That’s what sci­ence is all about.”


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A finding that many small galaxies don’t “swarm” around larger ones like bees but rather move around them in neat disc-shaped orbits is creating a new conundrum for scientists. Last year “we announced our startling discovery that half of the dwarf galaxies surrounding the Andromeda Galaxy are orbiting it in an immense plane,” said physicist Geraint Lewis from the University of Sydney in Australia. “This plane is more than a million light years in diameter, but is very thin, with a width of only 300,000 light years.” Now, astronomers are extending the finding to other galaxies. The universe contains billions of galaxies. Some, such as the Milky Way, are immense, containing hundreds of billions of stars. Most galaxies, however, are dwarfs, much smaller and with only a few billion stars. For decades astronomers have used computer models to predict how these dwarf galaxies should orbit large galaxies. They had always found that they should be scattered randomly. “Our Andromeda discovery did not agree with expectations, and we felt compelled to explore if it was true of other galaxies throughout the universe,” said Lewis. Using the Sloan Digital Sky Survey, a resource of color images and 3-D maps covering more than a third of the sky, the researchers dissected the properties of thousands of nearby galaxies. “We were surprised to find that a large proportion of pairs of satellite galaxies have oppositely directed velocities if they are situated on opposite sides of their giant galaxy hosts,” said Neil Ibata of the Lycée International in Strasbourg, France, lead author of a new study on the findings, published July 20 in the journal Nature. “Everywhere we looked we saw this strangely coherent coordinated motion of dwarf galaxies. From this we can extrapolate that these circular planes of dancing dwarfs are universal, seen in about 50 percent of galaxies,” said Lewis. “This is a big problem that contradicts our standard cosmological models. It challenges our understanding of how the universe works including the nature of dark matter,” an unseen material that is detected through its gravity. The researchers believe the answer may be hidden in some currently unknown physical process that governs how gas flows in the universe, although, as yet, there is no obvious mechanism that can guide dwarf galaxies into narrow planes. Some experts, however, have made more radical suggestions, including bending and twisting the laws of gravity and motion. “Throwing out seemingly established laws of physics is unpalatable,” said Professor Lewis, “but if our observations of nature are pointing us in this direction, we have to keep an open mind. That’s what science is all about.”