How galaxies get their swirl

  • Article by: MARK JOHNSON , Milwaukee Journal Sentinel
  • Updated: April 6, 2013 - 6:38 PM

Researchers revealed a new understanding on the spiral arms of galaxies in a step forward in explaining how the stars align.

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The Hubble Space Telescope photographed a group of galaxies called Arp 274, which is a system of three galaxies that appear to be partly overlapping in the image, although they may be at different distances. The spiral shapes of two of these galaxies appear mostly intact while the third, at left, is more compact.

Photo: NASA/Hubble Space Telescope via Associated Press file ,

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Pictures of the Milky Way, our own galaxy, show massive cloudy swirls of light spiraling inward toward a bright white disk. It’s a familiar image, even beyond our Milky Way. About 70 percent of galaxies feature similarly magnificent spiral arms. But the reasons galaxies take this pleasing form remain unknown.

Now a University of Wisconsin-Madison researcher and colleagues from the Harvard-Smithsonian Center for Astrophysics have used powerful computer simulations to gain a new understanding on the spiral arms in galaxies.

Their work, published in the Astrophysical Journal, shows for the first time that the spiral arms are permanent features.

“They are not transient. They do not come and go. They live forever,” said Elena D’Onghia, the paper’s lead author and an assistant professor in UW’s astronomy department.

The glimmering arms — made up of stars, gas and dust — are an effect of gravity and cause galaxies to look a little like the radar images we see of massive hurricanes. Studying this spiral pattern allows scientists to better understand how galaxies form and evolve and how stars align themselves under the influence of gravity.

“Gravity is the dominant force in galaxies,” D’Onghia said, adding there are still aspects of this force we don’t fully understand.

Her research suggests that giant molecular clouds help trigger the formation of the spiral arms and help sustain them.

How spiral arms develop has been a subject of keen interest among researchers, because “it’s one of the fundamental features of galaxies,” said Dawn Erb, an assistant professor in the department of physics at the University of Wisconsin, Milwaukee. “We want to understand how galaxies work and why they look the way they do.”

Erb was not involved in the new paper but called the work “a nice advance.”

While a majority of galaxies do have the spiral arms, Erb said that the feature is much rarer among more distant, less evolved galaxies.

D’Onghia says studying these features of the galaxy will help scientists monitor millions of stars in the Milky Way. She plans to work on developing more realistic models of galaxies. In particular, she is interested in looking closely at the effects gases have on the spiral arms.

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