Computer simulations show how spiral galaxies get their arms
Sen—Powerful new computer simulations are shedding light on the way spiral galaxies develop, and maintain, their distinctive arms.
Spiral galaxies are some of the most beautiful residents of the universe. Our own Milky Way is a spiral, with our solar system residing near one of its filament arms. Nearly 70 percent of the galaxies closest to the Milky Way are spirals, suggesting they have taken the most ordinary of galactic forms.
Yet how spiral galaxies get and maintain their characteristic arms has been an enduring puzzle in astrophysics. How do the arms of spiral galaxies arise? Do they change or come and go over time?
Two theories predominate. One holds that the arms come and go over time. A second theory is that the material that makes up 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.
Using powerful new computer simulations, a team of researchers from the University of Wisconsin-Madison and the Harvard-Smithsonian Center for Astrophysics followed the motions of as many as 100 million "stellar particles" as gravity and other astrophysical forces sculpted them, to resolve long-standing questions about the origin and life history of spiral arms in disk galaxies.
Computer simulation of a spiral galaxy. Image credit: Thiago Ize & Chris Johnson (Scientific Computing and Imaging Institute)
"We show for the first time that stellar spiral arms are not transient features, as claimed for several decades," says UW-Madison astrophysicist Elena D'Onghia, who led the new research along with Harvard colleagues Mark Vogelsberger and Lars Hernquist.
"The spiral arms are self-perpetuating, persistent, and surprisingly long lived," adds Vogelsberger.
The new results suggest that the arms arise in the first place as a result of the influence of giant molecular clouds, star forming regions or nurseries common in galaxies. Introduced into the simulation, the clouds act as "perturbers" and are enough to initiate the formation of spiral arms and to sustain them indefinitely.
"We find they are forming spiral arms," explains D'Onghia. "Past theory held the arms would go away with the perturbations removed, but we see that (once formed) the arms self-perpetuate, even when the perturbations are removed. It proves that once the arms are generated through these clouds, they can exist on their own through (the influence of) gravity, even in the extreme when the perturbations are no longer there."
The new study modelled stand-alone disk galaxies, those not influenced by another nearby galaxy or object. Some recent studies have explored the likelihood that spiral galaxies with a close neighbour (a nearby dwarf galaxy, for example) get their arms as gravity from the satellite galaxy pulls on the disk of its neighbour.
According to Vogelsberger and Hernquist, the new simulations can be used to reinterpret observational data, looking at both the high-density molecular clouds as well as gravitationally induced "holes" in space as the mechanisms that drive the formation of the characteristic arms of spiral galaxies.