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Astronomers working out how galaxies’ spiral arms form

April 2, 2013
Courtesy of UW-Madison
and World Science staff

Not all ga­lax­ies are spir­al-shaped. But those that are, ex­hibit­ing that ma­jes­tic, pin­wheel form that char­ac­ter­izes our gal­axy and oth­ers, are per­haps the most em­blem­at­ic of ga­lax­ies—the ga­lac­tic im­age that has most cap­tured the hu­man ima­gina­t­ion.

This im­age and the vi­deo an­i­ma­tion be­low show a sim­u­la­tion of arm for­ma­tion in spir­al ga­lax­ies. The vi­su­al­iza­tions were cre­at­ed by Thi­ago Ize and Chris John­son of the Uni­ver­si­ty of Utah’s Sci­en­tif­ic Com­put­ing and Im­ag­ing In­sti­tute.


Our so­lar sys­tem and Earth re­side some­where near one of the wispy, swept-back arms of the spir­al Milky Way gal­axy. And nearly 70 per­cent of the ga­lax­ies clos­est to us are spir­als, sug­gest­ing they have tak­en the most or­di­nary of ga­lac­tic forms in a uni­verse with bil­lions of ga­lax­ies.

But de­spite their com­mon shape, how ga­lax­ies like ours get and main­tain their char­ac­ter­is­tic arms has proved to be an en­dur­ing puz­zle. How do the arms arise? Do they change or come and go? 

The an­swers to these and oth­er ques­tions are com­ing in­to fo­cus as re­search­ers ex­ploit pow­er­ful new com­put­er sim­ula­t­ions to fol­low the mo­tions of as many as 100 mil­lion “stel­lar par­t­i­cles” or sim­ulated stars as gra­vity and oth­er as­t­ro­phys­i­cal forc­es sculpt them in­to fa­mil­iar ga­lac­tic shapes. 

Writ­ing April 1 in The As­t­ro­phys­i­cal Jour­nal, a team of re­search­ers re­port sim­ula­t­ions that they say seem to re­solve long­stand­ing ques­tions about the or­i­gin and life course of the arms.

“We show for the first time that stel­lar spir­al arms are not tran­sient fea­tures, as claimed for sev­er­al dec­ades,” said Uni­vers­ity of Wisconsin-Madison as­t­ro­phys­i­cist El­e­na D’Onghia, who led the re­search with Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics col­leagues Mark Vo­gels­berger and Lars Hern­quist. “They are self-per­pet­u­ating, per­sist­ent and sur­pris­ingly long lived.”

Two the­o­ries on spir­al arms have pre­dom­i­nat­ed. One holds that they come and go. A sec­ond, widely held idea is that the ma­te­ri­al in the arms – stars, gas and dust – is af­fect­ed by dif­fer­ences in gra­vity and jams up, like cars at rush hour, sus­tain­ing the arms for long pe­ri­ods.

The new find­ings fall some­where in be­tween the two the­o­ries and sug­gest that the arms or­i­ginate thanks to the in­flu­ence of gi­ant mo­lec­u­lar clouds, star form­ing re­gions or stel­lar “nurs­eries” com­mon in ga­lax­ies, the re­search­ers said. In­tro­duced in­to the sim­ula­t­ion, the clouds, said D’Onghia, act as “per­turbers” that are enough to not only trig­ger spir­al arm forma­t­ion but to sus­tain the arms in­def­i­nite­ly.

Click to play animation


“Past the­o­ry held the arms would go away with the per­turba­t­ions re­moved, but we see that the arms self-perpetuate, even when the per­turba­t­ions are re­moved,” she ex­plained. “It proves that once the arms are gen­er­at­ed through these clouds, they can ex­ist on their own through [the in­flu­ence of] gra­vity.”

The study mod­eled iso­lat­ed ga­lax­ies. Some re­cent stud­ies have ex­plored the like­li­hood that spir­al ga­lax­ies with a close neigh­bor — a near­by dwarf gal­axy, for ex­am­ple — get their arms as gra­vity from the sat­el­lite gal­axy pulls on the disk of its neigh­bor.

Ac­cord­ing to Vo­gels­berger and Hern­quist, the new sim­ula­t­ions can be used to re­in­ter­pret ob­serva­t­ions, look­ing at both the thick mo­lec­u­lar clouds as well as gravita­t­ionally in­duced “holes” in ga­lac­tic ma­te­ri­al as mech­a­nisms driv­ing the forma­t­ion of the spir­al arms.


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Not all galaxies are spiral-shaped. But those that are, exhibiting that majestic, pinwheel form that characterizes our galaxy and others, are perhaps the most emblematic of galaxies—the galactic image that has most captured the human imagination. Our solar system and Earth reside somewhere near one of the wispy, swept-back arms of the spiral Milky Way galaxy. And nearly 70 percent of the galaxies closest to us are spirals, suggesting they have taken the most ordinary of galactic forms in a universe with billions of galaxies. But despite their common shape, how galaxies like ours get and maintain their characteristic arms has proved to be an enduring puzzle. How do the arms arise? Do they change or come and go? The answers to these and other questions are coming into focus as researchers exploit powerful new computer simulations to follow the motions of as many as 100 million “stellar particles” or simulated stars as gravity and other astrophysical forces sculpt them into familiar galactic shapes. Writing April 1 in The Astrophysical Journal, a team of researchers report simulations that they say seem to resolve longstanding questions about the origin and life course of the arms. “We show for the first time that stellar spiral arms are not transient features, as claimed for several decades,” said University of Wisconsin-Madison astrophysicist Elena D’Onghia, who led the research with Harvard-Smithsonian Center for Astrophysics colleagues Mark Vogelsberger and Lars Hernquist. “They are self-perpetuating, persistent and surprisingly long lived.” Two theories on spiral arms have predominated. One holds that they come and go. A second, widely held idea is that the material in the arms – stars, gas and dust – is affected by differences in gravity and jams up, like cars at rush hour, sustaining the arms for long periods. The new findings fall somewhere in between the two theories and suggest that the arms originate thanks to the influence of giant molecular clouds, star forming regions or stellar “nurseries” common in galaxies, the researchers said. Introduced into the simulation, the clouds, said D’Onghia, act as “perturbers” that are enough to not only trigger spiral arm formation but to sustain the arms indefinitely. “Past theory held the arms would go away with the perturbations removed, but we see that the arms self-perpetuate, even when the perturbations are removed,” she explained. “It proves that once the arms are generated through these clouds, they can exist on their own through [the influence of] gravity.” The new study modeled isolated disk galaxies. Some recent studies have explored the likelihood that spiral galaxies with a close neighbor — a nearby dwarf galaxy, for example — get their arms as gravity from the satellite galaxy pulls on the disk of its neighbor. According to Vogelsberger and Hernquist, the new simulations can be used to reinterpret observations, looking at both the thick molecular clouds as well as gravitationally induced “holes” in galactic material as mechanisms driving the formation of the spiral arms.