How Spiral Galaxies Get Their Arms
April 3, 2013

Simulations Offer New Insight On How Spiral Galaxies Get Their Arms

Lee Rannals for - Your Universe Online

Astronomers writing in The Astrophysical Journal have answered a few more important questions about how so-called spiral galaxies get their arms.

The team used powerful computer simulations to follow the motions of as many as 100 million "stellar particles" as gravity and other astrophysical forces began to give them shape. Their simulations have answered some long-standing questions about the origin and life history of spiral arms in disk galaxies.

"We show for the first time that stellar spiral arms are not transient features, as claimed for several decades," said UW-Madison astrophysicist Elena D'Onghia in a statement.

Mark Vogelsberger of the Harvard-Smithsonian Center for Astrophysics added that the spiral arms are self-perpetuating, persistent and surprisingly long lived.

One theory for the origin and fate of the iconic spiral arms is that they appear and disappear over time. A second theory is that the material that makes up the arms is affected by differences in gravity and jams up, sustaining the arms for long periods.

Results from the new study show that the answer may actually be a combination of both theories, suggesting that the arms arise as a result of the influence of giant molecular clouds. These clouds act as "perturbers" and are enough to not only initiate the formation of spiral arms but to sustain them indefinitely.

"We find they are forming spiral arms," said 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."

Recent studies have explored the likelihood that spiral galaxies with close neighbors get their arms as gravity from the satellite galaxy pulls on the disk of its neighbor. However, the new study modeled stand-alone disk galaxies rather than those that might be influenced by another nearby galaxy.

The researchers say 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.

In January astronomers reported that they discovered that a previously known galaxy is actually the largest known spiral galaxy in the entire universe. NGC 6827 is about 212 million light-years from Earth, and is about 522,000 light-years in diameter — more than five times the size of our own galaxy.